To address your second point, you seem to be saying that missing the frame budget once and then compositing the rest of the time would be better than missing th

To address your second point, you seem to be saying that missing the frame budget once and then compositing the rest of the time would be better than missing the frame budget every time. That is certainly true, but a) the cases where you can do everything as a compositor optimization are very few (transform and opacity mostly) so aside from a few fast paths you'd miss your frame budget all the time there too, and b) we have a lot of examples of web pages that are slow on CPU renderers and very fast on WebRender and very few examples of the opposite aside from constructed edge case benchmarks. Those we have found had solutions and I suspect the other cases will too. As resolution and framerate scale, CPUs cannot keep up. GPUs are the only practical path forward.