diff --git a/package.json b/package.json
index d7ff4a28..c7eb38d5 100644
--- a/package.json
+++ b/package.json
@@ -41,7 +41,8 @@
     "test:e2e:ui": "npm run build:main && npm run build:renderer && playwright test --ui",
     "test:e2e:headed": "npm run build:main && npm run build:renderer && playwright test --headed",
     "test:integration": "vitest run --config vitest.integration.config.ts",
-    "test:integration:watch": "vitest --config vitest.integration.config.ts"
+    "test:integration:watch": "vitest --config vitest.integration.config.ts",
+    "test:performance": "vitest run --config vitest.performance.config.mts"
   },
   "build": {
     "appId": "com.maestro.app",
diff --git a/src/__tests__/performance/ThinkingStreamPerformance.test.tsx b/src/__tests__/performance/ThinkingStreamPerformance.test.tsx
new file mode 100644
index 00000000..9a4fa27f
--- /dev/null
+++ b/src/__tests__/performance/ThinkingStreamPerformance.test.tsx
@@ -0,0 +1,709 @@
+/**
+ * @file ThinkingStreamPerformance.test.tsx
+ * @description Performance tests for the Show Thinking feature with large streams
+ *
+ * Task 6.5 - Test performance with large thinking streams (10-50KB+ per response):
+ * - RAF throttling efficiency for rapid chunk arrivals
+ * - Memory usage during large stream accumulation
+ * - UI responsiveness with 10KB, 25KB, and 50KB+ thinking content
+ * - Chunk batching effectiveness
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { render, screen, waitFor, act } from '@testing-library/react';
+import React from 'react';
+import { LayerStackProvider } from '../../renderer/contexts/LayerStackContext';
+import type { Theme, LogEntry, Session, AITab } from '../../renderer/types';
+
+// ============================================================================
+// Test Utilities
+// ============================================================================
+
+/**
+ * Generate a large thinking stream content of specified size
+ * Simulates Codex reasoning tokens which can be very verbose
+ */
+const generateThinkingContent = (sizeKb: number): string => {
+  const targetBytes = sizeKb * 1024;
+  const reasoningPatterns = [
+    'Let me analyze this step by step. ',
+    'First, I need to understand the context. ',
+    'Looking at the code structure, I can see that ',
+    'The implementation requires considering several factors: ',
+    'Based on my analysis, the approach should be ',
+    'Examining the dependencies and their interactions... ',
+    'This function handles the core logic for ',
+    'The data flow follows this pattern: ',
+    'Considering edge cases such as null values and errors... ',
+    'The optimal solution would involve ',
+  ];
+
+  let content = '';
+  let patternIndex = 0;
+
+  while (content.length < targetBytes) {
+    content += reasoningPatterns[patternIndex % reasoningPatterns.length];
+    patternIndex++;
+  }
+
+  return content.slice(0, targetBytes);
+};
+
+/**
+ * Split content into chunks of varying sizes (simulating real streaming)
+ */
+const splitIntoChunks = (content: string, avgChunkSize: number): string[] => {
+  const chunks: string[] = [];
+  let position = 0;
+
+  while (position < content.length) {
+    // Vary chunk size by ±50% to simulate real network conditions
+    const variation = 0.5 + Math.random();
+    const chunkSize = Math.floor(avgChunkSize * variation);
+    chunks.push(content.slice(position, position + chunkSize));
+    position += chunkSize;
+  }
+
+  return chunks;
+};
+
+// Create mock theme
+const createMockTheme = (): Theme => ({
+  id: 'test-theme',
+  name: 'Test Theme',
+  mode: 'dark',
+  colors: {
+    bgMain: '#1a1a1a',
+    bgPanel: '#252525',
+    bgActivity: '#2d2d2d',
+    textMain: '#ffffff',
+    textDim: '#888888',
+    accent: '#0066ff',
+    accentText: '#4488ff',
+    accentForeground: '#ffffff',
+    border: '#333333',
+    highlight: '#0066ff33',
+    success: '#00aa00',
+    warning: '#ffaa00',
+    error: '#ff0000',
+  },
+});
+
+// Mock the thinking chunk handler logic (extracted from App.tsx)
+interface ThinkingChunkBuffer {
+  buffer: Map<string, string>;
+  rafId: number | null;
+}
+
+const createThinkingChunkHandler = (
+  onUpdate: (sessionId: string, tabId: string, content: string) => void
+) => {
+  const state: ThinkingChunkBuffer = {
+    buffer: new Map(),
+    rafId: null,
+  };
+
+  const handleChunk = (sessionId: string, tabId: string, content: string) => {
+    const bufferKey = `${sessionId}:${tabId}`;
+    const existingContent = state.buffer.get(bufferKey) || '';
+    state.buffer.set(bufferKey, existingContent + content);
+
+    if (state.rafId === null) {
+      state.rafId = requestAnimationFrame(() => {
+        const chunksToProcess = new Map(state.buffer);
+        state.buffer.clear();
+        state.rafId = null;
+
+        for (const [key, bufferedContent] of chunksToProcess) {
+          const [sid, tid] = key.split(':');
+          onUpdate(sid, tid, bufferedContent);
+        }
+      });
+    }
+  };
+
+  const cleanup = () => {
+    if (state.rafId !== null) {
+      cancelAnimationFrame(state.rafId);
+      state.rafId = null;
+    }
+    state.buffer.clear();
+  };
+
+  return { handleChunk, cleanup, getBufferSize: () => state.buffer.size };
+};
+
+// ============================================================================
+// Performance Test Component
+// ============================================================================
+
+interface ThinkingDisplayProps {
+  logs: LogEntry[];
+  theme: Theme;
+}
+
+const ThinkingDisplay: React.FC<ThinkingDisplayProps> = ({ logs, theme }) => {
+  const thinkingLogs = logs.filter(l => l.source === 'thinking');
+
+  return (
+    <div data-testid="thinking-display">
+      {thinkingLogs.map(log => (
+        <div
+          key={log.id}
+          data-testid="thinking-entry"
+          className="px-4 py-2 text-sm font-mono border-l-2"
+          style={{
+            color: theme.colors.textDim,
+            borderColor: theme.colors.accentText,
+            backgroundColor: `${theme.colors.accentText}05`,
+            opacity: 0.85,
+          }}
+        >
+          <div className="flex items-center gap-2 mb-1">
+            <span
+              className="text-[10px] px-1.5 py-0.5 rounded"
+              style={{
+                backgroundColor: `${theme.colors.accentText}20`,
+                color: theme.colors.accentText
+              }}
+            >
+              thinking
+            </span>
+          </div>
+          <div className="whitespace-pre-wrap" data-testid="thinking-content">
+            {log.text}
+          </div>
+        </div>
+      ))}
+    </div>
+  );
+};
+
+// ============================================================================
+// Tests
+// ============================================================================
+
+describe('ThinkingStreamPerformance', () => {
+  let mockRaf: (callback: FrameRequestCallback) => number;
+  let mockCancelRaf: (id: number) => void;
+  let rafCallbacks: Map<number, FrameRequestCallback>;
+  let rafIdCounter: number;
+
+  beforeEach(() => {
+    vi.useFakeTimers();
+
+    // Set up RAF mock with control over when frames execute
+    rafCallbacks = new Map();
+    rafIdCounter = 0;
+
+    mockRaf = vi.fn((callback: FrameRequestCallback) => {
+      const id = ++rafIdCounter;
+      rafCallbacks.set(id, callback);
+      return id;
+    });
+
+    mockCancelRaf = vi.fn((id: number) => {
+      rafCallbacks.delete(id);
+    });
+
+    // Replace global functions
+    global.requestAnimationFrame = mockRaf;
+    global.cancelAnimationFrame = mockCancelRaf;
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+    rafCallbacks.clear();
+  });
+
+  // Helper to flush all pending RAF callbacks
+  const flushRafCallbacks = () => {
+    const callbacks = Array.from(rafCallbacks.values());
+    rafCallbacks.clear();
+    const timestamp = performance.now();
+    callbacks.forEach(cb => cb(timestamp));
+  };
+
+  describe('RAF Throttling Efficiency', () => {
+    it('should batch multiple rapid chunk arrivals into single RAF callback', () => {
+      const updates: Array<{ sessionId: string; tabId: string; content: string }> = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (sessionId, tabId, content) => updates.push({ sessionId, tabId, content })
+      );
+
+      // Simulate 100 rapid chunks arriving within the same frame
+      const chunks = splitIntoChunks(generateThinkingContent(10), 100);
+
+      for (const chunk of chunks) {
+        handleChunk('session-1', 'tab-1', chunk);
+      }
+
+      // Before RAF fires, no updates should have happened
+      expect(updates.length).toBe(0);
+      expect(mockRaf).toHaveBeenCalledTimes(1); // Only one RAF scheduled
+
+      // Execute the RAF callback
+      flushRafCallbacks();
+
+      // All chunks should be batched into a single update
+      expect(updates.length).toBe(1);
+      expect(updates[0].content.length).toBeGreaterThan(chunks.length * 50); // Batched content
+
+      cleanup();
+    });
+
+    it('should handle chunks for multiple sessions simultaneously', () => {
+      const updates: Array<{ sessionId: string; tabId: string; content: string }> = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (sessionId, tabId, content) => updates.push({ sessionId, tabId, content })
+      );
+
+      // Send chunks to 3 different sessions
+      handleChunk('session-1', 'tab-1', 'Content for session 1');
+      handleChunk('session-2', 'tab-1', 'Content for session 2');
+      handleChunk('session-3', 'tab-1', 'Content for session 3');
+      handleChunk('session-1', 'tab-1', ' - more content'); // Additional for session 1
+
+      expect(mockRaf).toHaveBeenCalledTimes(1);
+
+      flushRafCallbacks();
+
+      // Should have 3 updates (one per session)
+      expect(updates.length).toBe(3);
+
+      const session1Update = updates.find(u => u.sessionId === 'session-1');
+      expect(session1Update?.content).toBe('Content for session 1 - more content');
+
+      cleanup();
+    });
+
+    it('should not schedule new RAF while one is pending', () => {
+      const updates: Array<{ sessionId: string; tabId: string; content: string }> = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (sessionId, tabId, content) => updates.push({ sessionId, tabId, content })
+      );
+
+      // Send many chunks
+      for (let i = 0; i < 1000; i++) {
+        handleChunk('session-1', 'tab-1', `chunk-${i} `);
+      }
+
+      // Should only have one RAF scheduled despite 1000 chunks
+      expect(mockRaf).toHaveBeenCalledTimes(1);
+
+      flushRafCallbacks();
+
+      expect(updates.length).toBe(1);
+      expect(updates[0].content).toContain('chunk-0');
+      expect(updates[0].content).toContain('chunk-999');
+
+      cleanup();
+    });
+  });
+
+  describe('Large Stream Handling', () => {
+    it('should handle 10KB thinking stream efficiently', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      const content = generateThinkingContent(10);
+      const chunks = splitIntoChunks(content, 256); // Average 256 bytes per chunk
+
+      const startTime = performance.now();
+
+      for (const chunk of chunks) {
+        handleChunk('session-1', 'tab-1', chunk);
+      }
+
+      flushRafCallbacks();
+
+      const elapsed = performance.now() - startTime;
+
+      // Performance assertion: should process 10KB in under 100ms
+      expect(elapsed).toBeLessThan(100);
+      expect(updates.length).toBe(1);
+      expect(updates[0].length).toBe(content.length);
+
+      cleanup();
+    });
+
+    it('should handle 25KB thinking stream efficiently', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      const content = generateThinkingContent(25);
+      const chunks = splitIntoChunks(content, 512);
+
+      const startTime = performance.now();
+
+      for (const chunk of chunks) {
+        handleChunk('session-1', 'tab-1', chunk);
+      }
+
+      flushRafCallbacks();
+
+      const elapsed = performance.now() - startTime;
+
+      // Performance assertion: should process 25KB in under 150ms
+      expect(elapsed).toBeLessThan(150);
+      expect(updates.length).toBe(1);
+      expect(updates[0].length).toBe(content.length);
+
+      cleanup();
+    });
+
+    it('should handle 50KB thinking stream (Codex reasoning) efficiently', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      const content = generateThinkingContent(50);
+      const chunks = splitIntoChunks(content, 1024);
+
+      const startTime = performance.now();
+
+      for (const chunk of chunks) {
+        handleChunk('session-1', 'tab-1', chunk);
+      }
+
+      flushRafCallbacks();
+
+      const elapsed = performance.now() - startTime;
+
+      // Performance assertion: should process 50KB in under 200ms
+      expect(elapsed).toBeLessThan(200);
+      expect(updates.length).toBe(1);
+      expect(updates[0].length).toBe(content.length);
+
+      cleanup();
+    });
+
+    it('should handle 100KB+ extreme stream without hanging', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      const content = generateThinkingContent(100);
+      const chunks = splitIntoChunks(content, 2048);
+
+      const startTime = performance.now();
+
+      for (const chunk of chunks) {
+        handleChunk('session-1', 'tab-1', chunk);
+      }
+
+      flushRafCallbacks();
+
+      const elapsed = performance.now() - startTime;
+
+      // Performance assertion: should process 100KB in under 500ms
+      expect(elapsed).toBeLessThan(500);
+      expect(updates.length).toBe(1);
+      expect(updates[0].length).toBe(content.length);
+
+      cleanup();
+    });
+  });
+
+  describe('Memory Efficiency', () => {
+    it('should clear buffer after processing', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup, getBufferSize } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      handleChunk('session-1', 'tab-1', 'test content');
+      expect(getBufferSize()).toBe(1);
+
+      flushRafCallbacks();
+
+      expect(getBufferSize()).toBe(0);
+
+      cleanup();
+    });
+
+    it('should cleanup properly on unmount', () => {
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        () => {}
+      );
+
+      // Schedule some chunks
+      handleChunk('session-1', 'tab-1', 'test');
+
+      // Cleanup before RAF fires
+      cleanup();
+
+      // RAF should have been cancelled
+      expect(mockCancelRaf).toHaveBeenCalled();
+    });
+
+    it('should not accumulate memory with repeated stream cycles', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup, getBufferSize } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      // Simulate multiple complete stream cycles
+      for (let cycle = 0; cycle < 10; cycle++) {
+        const content = generateThinkingContent(5);
+        const chunks = splitIntoChunks(content, 512);
+
+        for (const chunk of chunks) {
+          handleChunk('session-1', 'tab-1', chunk);
+        }
+
+        flushRafCallbacks();
+
+        // Buffer should be empty after each cycle
+        expect(getBufferSize()).toBe(0);
+      }
+
+      expect(updates.length).toBe(10);
+
+      cleanup();
+    });
+  });
+
+  describe('UI Rendering Performance', () => {
+    it('should render 10KB thinking content without performance issues', () => {
+      const theme = createMockTheme();
+      const content = generateThinkingContent(10);
+      const logs: LogEntry[] = [{
+        id: 'thinking-1',
+        timestamp: Date.now(),
+        source: 'thinking',
+        text: content,
+      }];
+
+      const startTime = performance.now();
+
+      const { container } = render(
+        <LayerStackProvider>
+          <ThinkingDisplay logs={logs} theme={theme} />
+        </LayerStackProvider>
+      );
+
+      const elapsed = performance.now() - startTime;
+
+      // Should render in under 100ms
+      expect(elapsed).toBeLessThan(100);
+
+      const thinkingContent = screen.getByTestId('thinking-content');
+      expect(thinkingContent.textContent?.length).toBe(content.length);
+    });
+
+    it('should render 50KB thinking content without hanging', () => {
+      const theme = createMockTheme();
+      const content = generateThinkingContent(50);
+      const logs: LogEntry[] = [{
+        id: 'thinking-1',
+        timestamp: Date.now(),
+        source: 'thinking',
+        text: content,
+      }];
+
+      const startTime = performance.now();
+
+      render(
+        <LayerStackProvider>
+          <ThinkingDisplay logs={logs} theme={theme} />
+        </LayerStackProvider>
+      );
+
+      const elapsed = performance.now() - startTime;
+
+      // Should render in under 500ms even for large content
+      expect(elapsed).toBeLessThan(500);
+    });
+
+    it('should handle incremental content updates efficiently', async () => {
+      const theme = createMockTheme();
+      const logs: LogEntry[] = [{
+        id: 'thinking-1',
+        timestamp: Date.now(),
+        source: 'thinking',
+        text: 'Initial content',
+      }];
+
+      const { rerender } = render(
+        <LayerStackProvider>
+          <ThinkingDisplay logs={logs} theme={theme} />
+        </LayerStackProvider>
+      );
+
+      // Simulate incremental updates (like streaming)
+      const updateTimes: number[] = [];
+
+      for (let i = 0; i < 20; i++) {
+        const startTime = performance.now();
+
+        // Append more content
+        logs[0].text += generateThinkingContent(1);
+
+        rerender(
+          <LayerStackProvider>
+            <ThinkingDisplay logs={[{ ...logs[0] }]} theme={theme} />
+          </LayerStackProvider>
+        );
+
+        updateTimes.push(performance.now() - startTime);
+      }
+
+      // Average update time should be under 50ms
+      const avgTime = updateTimes.reduce((a, b) => a + b, 0) / updateTimes.length;
+      expect(avgTime).toBeLessThan(50);
+    });
+  });
+
+  describe('Chunk Batching Edge Cases', () => {
+    it('should handle empty chunks gracefully', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      handleChunk('session-1', 'tab-1', '');
+      handleChunk('session-1', 'tab-1', 'actual content');
+      handleChunk('session-1', 'tab-1', '');
+
+      flushRafCallbacks();
+
+      expect(updates.length).toBe(1);
+      expect(updates[0]).toBe('actual content');
+
+      cleanup();
+    });
+
+    it('should handle very small chunks (1-5 bytes) efficiently', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      const content = generateThinkingContent(5);
+
+      // Split into very small chunks (simulating character-by-character streaming)
+      for (let i = 0; i < content.length; i++) {
+        handleChunk('session-1', 'tab-1', content[i]);
+      }
+
+      expect(mockRaf).toHaveBeenCalledTimes(1); // Still just one RAF
+
+      flushRafCallbacks();
+
+      expect(updates.length).toBe(1);
+      expect(updates[0]).toBe(content);
+
+      cleanup();
+    });
+
+    it('should handle interleaved chunks from multiple tabs', () => {
+      const updates: Array<{ sessionId: string; tabId: string; content: string }> = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (sessionId, tabId, content) => updates.push({ sessionId, tabId, content })
+      );
+
+      // Interleave chunks from different tabs
+      for (let i = 0; i < 100; i++) {
+        handleChunk('session-1', `tab-${i % 3}`, `chunk-${i} `);
+      }
+
+      flushRafCallbacks();
+
+      // Should have 3 updates (one per tab)
+      expect(updates.length).toBe(3);
+
+      // Verify each tab got its chunks
+      const tab0Update = updates.find(u => u.tabId === 'tab-0');
+      const tab1Update = updates.find(u => u.tabId === 'tab-1');
+      const tab2Update = updates.find(u => u.tabId === 'tab-2');
+
+      expect(tab0Update?.content).toContain('chunk-0');
+      expect(tab1Update?.content).toContain('chunk-1');
+      expect(tab2Update?.content).toContain('chunk-2');
+
+      cleanup();
+    });
+  });
+
+  describe('Stress Testing', () => {
+    it('should handle sustained high-frequency chunk arrivals', () => {
+      const updates: string[] = [];
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (_sid, _tid, content) => updates.push(content)
+      );
+
+      // Simulate 10 seconds of sustained streaming at 60fps
+      // Each frame gets 10 chunks
+      const framesCount = 600; // 10 seconds at 60fps
+      const chunksPerFrame = 10;
+
+      const startTime = performance.now();
+
+      for (let frame = 0; frame < framesCount; frame++) {
+        for (let chunk = 0; chunk < chunksPerFrame; chunk++) {
+          handleChunk('session-1', 'tab-1', `frame-${frame}-chunk-${chunk} `);
+        }
+
+        // Flush RAF to simulate frame completion
+        flushRafCallbacks();
+      }
+
+      const elapsed = performance.now() - startTime;
+
+      // Should process all frames in reasonable time (under 5 seconds with fake timers)
+      expect(elapsed).toBeLessThan(5000);
+      expect(updates.length).toBe(framesCount);
+
+      cleanup();
+    });
+
+    it('should maintain consistency under concurrent session load', () => {
+      const updates: Map<string, string[]> = new Map();
+      const { handleChunk, cleanup } = createThinkingChunkHandler(
+        (sessionId, _tabId, content) => {
+          const sessionUpdates = updates.get(sessionId) || [];
+          sessionUpdates.push(content);
+          updates.set(sessionId, sessionUpdates);
+        }
+      );
+
+      const sessionCount = 10;
+      const chunksPerSession = 100;
+
+      // Send chunks to many sessions
+      for (let chunk = 0; chunk < chunksPerSession; chunk++) {
+        for (let session = 0; session < sessionCount; session++) {
+          handleChunk(`session-${session}`, 'tab-1', `s${session}c${chunk} `);
+        }
+
+        // Flush every 10 chunks
+        if ((chunk + 1) % 10 === 0) {
+          flushRafCallbacks();
+        }
+      }
+
+      // Final flush
+      flushRafCallbacks();
+
+      // Each session should have received all its chunks
+      for (let session = 0; session < sessionCount; session++) {
+        const sessionUpdates = updates.get(`session-${session}`);
+        expect(sessionUpdates).toBeDefined();
+
+        // Combine all updates for this session
+        const fullContent = sessionUpdates!.join('');
+        expect(fullContent).toContain(`s${session}c0`);
+        expect(fullContent).toContain(`s${session}c99`);
+      }
+
+      cleanup();
+    });
+  });
+});
diff --git a/vitest.performance.config.mts b/vitest.performance.config.mts
new file mode 100644
index 00000000..abfb1298
--- /dev/null
+++ b/vitest.performance.config.mts
@@ -0,0 +1,31 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+import path from 'path';
+
+/**
+ * Performance test configuration
+ * Run with: npx vitest run --config vitest.performance.config.mts
+ */
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    setupFiles: ['./src/__tests__/setup.ts'],
+    // Only include performance tests
+    include: ['src/__tests__/performance/**/*.{test,spec}.{ts,tsx}'],
+    exclude: [
+      'node_modules',
+      'dist',
+      'release',
+    ],
+    testTimeout: 30000, // Longer timeout for performance tests
+    hookTimeout: 10000,
+    teardownTimeout: 5000,
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './src'),
+    },
+  },
+});