There is not absolute rule stating that layout and painting must be slow while compositing is not. In fact Mozillians are currently working on a new rendering pipeline[1] based on state of the art rendering tools[2]. This new pipeline should be deployed in firefox in 2018 as a part of the ongoing Quantum project.

[1] https://github.com/servo/webrender

[2] https://m.youtube.com/watch?v=BTURkjYJ_uk