The path to better performance is often found in simplicity.

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GitHub's engineering team details how they optimized the pull request Files Changed tab by refactoring React diff-line components. The v1 architecture had 8–13 React components and 20+ event handlers per diff line, causing JavaScript heaps exceeding 1 GB and 400,000+ DOM nodes on large PRs. The v2 redesign reduced components from 8 to 2 per line, adopted O(1) data lookups via JavaScript Maps, moved complex state to conditionally rendered children, and replaced scattered useEffect hooks with top-level handlers using data attributes. Results on a 10,000-line diff: 74% fewer rendered components, ~50% less memory, and ~78% faster INP. For p95+ pull requests (10,000+ diff lines), TanStack Virtual was integrated for window virtualization, achieving a 10x reduction in heap and DOM nodes, with INP dropping from 275–700ms to 40–80ms. Additional wins came from replacing heavy CSS selectors, GPU-accelerated drag/resize, server-side hydration of only visible lines, and progressive diff loading.

The uphill climb of making diff lines performant

Performance improvements by pull request size and complexity

Virtualization for our largest pull requests

Diff-initely better: The power of streamlined performance