The Layers panel chose between the UI thread and the thumbnail thread
for a row's first thumbnail by counting objects only, so one path of
200,000 anchors was rendered on the UI thread. A row whose paths hold
more than 2,000 anchors in all now goes to the thumbnail thread, as a
row of more than 200 objects already did.
While such a row waits, the painted fallback thumbnail draws nothing
for it instead of measuring the path's bounds on every frame.
Opening the panel on one path of 200,000 anchors: the first frame takes
2.4 ms instead of 393 ms (release build, median of three runs).
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