When comparing root versus overflow scroll element performance on the tests below, the element version uses almost double the main thread CPU time.

root: /src/chrome/test/data/perf/tough_compositor_cases/infinite_scroll_root_n_layers.html?layer_count=306

element: /src/chrome/test/data/perf/tough_compositor_cases/infinite_scroll_element_n_layers.html?layer_count=306

Part of the reason is that for non-root layers, we set SetNeedsCompositingUpdate on the layer in PaintLayerScrollableArea::UpdateCompositingLayersAfterScroll().