It depends. For a lot of web pages, the things webrender do are actually very simple and can be faster than Firefox's compositor even if there was no painting required. The expensive stuff like rasterizing glyphs are cached. The idea is that if we can render everything at a cost that is equivalent to that of the compositor, then the benefits of the compositing infrastructure aren't clear anymore. For sure there will be pages where the compositor would have been faster for frames where no painting was required, at the end of the day it's a new architecture with new tradeoffs, but the goal is to be overall (much) faster.