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One layer per item instead of one per moving part doubled a web drag's p95: 3D-turned pictures need their own

Rule

If Layerize dominates a web frame with many layers, do not merge an item's 3D-turned pictures into one layer: measure first, since the turned pictures then repaint at full DPR inside it.

Snippet
// tried: one layer for the whole piece (its root only)
root: { position: 'absolute', willChange: 'transform' },
clip: { position: 'absolute', overflow: 'hidden' },   // no layer
// kept: every moving box its own layer; only still boxes have none
root: { position: 'absolute', willChange: 'transform, opacity' },
clip: { position: 'absolute', overflow: 'hidden', willChange: 'transform, opacity' },
Check

trace the same drag with --cpu 4 --dpr 3 both ways: with one layer per piece the drag's p95 doubles (here 94 → 185 ms, a step 432 ms).

Applies to

react-native-web@>=0.21 <0.22, other: Chrome with GPU compositing

I assumed

with Layerize the largest part of a 4× CPU frame (18.3 ms of it, with script 13.3, commit 6.3 and style 5.0), fewer layers would be cheaper.

What happened

one layer per piece instead of five made the drag's p95 go from 94 to 185 ms and a step to 432 ms at 4× CPU and 3× DPR.

Why

the turning photos, with their perspective transforms, repainted at 3× inside the piece's single layer every frame instead of being moved by the compositor.

The fix

reverted; each moving box keeps its layer, and each moving tag and thread got one too (without, the 28 of them repainted the rail's layer and the drag's p95 at 1× rose from 21 to 37 ms).

Evidence

the paired runs of the polish round (commit a22aa93).

Learned in the articleA garment rail in React Native where each piece turns on its hook: perspective, bunching, swing

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