perf(site): use lazy iteration in sliceAtGraphemeBoundary#23671
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perf(site): use lazy iteration in sliceAtGraphemeBoundary#23671
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Array.from(graphemeSegmenter.segment(text)) materializes the entire text into an array before iterating, even though the loop breaks early at the visible prefix length. During streaming at 60fps, this makes each frame O(full text) instead of O(prefix). Benchmark on 5000-char text with 200-char prefix: 22.6x faster (1.44ms to 0.06ms per call, saving 8.3% of the frame budget). The fallback codepoint path had the same issue with Array.from.
DanielleMaywood
approved these changes
Mar 26, 2026
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Summary
Fix O(full-text) per-frame cost in the streaming text smooth-reveal animation by replacing eager array materialization with lazy iteration.
What changed
sliceAtGraphemeBoundaryinSmoothText.tsusedArray.from(graphemeSegmenter.segment(text))to materialize the entire grapheme segment iterable into an array before iterating with an earlybreak. The fallback codepoint path had the same issue withArray.from(text).During streaming, this function runs on every animation frame (~60fps) for the actively streaming response block. The visible prefix is typically much shorter than the accumulated full text, so the early
breakshould make the function O(prefix), but the eagerArray.fromforces O(full-text) regardless.Replaced both paths with direct
for...ofiteration, which is lazy and exits at thebreak.Benchmark results
Frame budget is measured against 16.7ms (60fps). At 10K characters (a medium-length response), the old code consumed 17.66% of the frame budget on grapheme slicing alone.
Verification
SmoothText.test.tstests pass.lint:compilerreports 181 functions compiled, 0 diagnostics.useSmoothStreamingTexthook still compiles cleanly.