Each 30fps streaming-reveal tick re-segmented the whole revealed prefix via sliceGraphemes (per-tick slice cost grew with the prefix). BlockUnitCounter already memoized per-block grapheme counts; apply the same idea to slicing: cache the slice boundary per block index and re-segment only the per-step delta from the boundary cluster, so per-update cost grows with the delta instead of the rendered prefix.
buildDisplayMessage gains an optional sliceOf seam (default = sliceGraphemes, so existing callers are unchanged); the controller wires its BlockUnitCounter.slice through it via a #build helper that also de-duplicates six previously-repeated buildDisplayMessage call sites.
Only an exact (text, units) hit skips segmentation; the incremental guard (text === cached.text || text.startsWith(cached.text)) && units >= cached.units re-segments from the boundary cluster, so an append that extends the final cluster (e.g. a -> a combining acute, ZWJ family merge) is never stale.
Benchmark (bench/streaming-throughput.bench.ts, 30520-grapheme message, 61 ticks/episode): 23.78ms -> 2.27ms per episode (~10x). Regression tests vs a pure Intl.Segmenter reference cover fixed-text growing units, append growth, boundary-cluster extension, multi-block indices, shrink/regrow, full replace, and a 400-step seeded fuzz.
oldschoola
718c7cea2f