No, but somehow I get the feeling we're talking about entirely different levels of "slower" here.
You're talking about GPU-accelerating complicated page renders.
I'm talking about things like regularly seeing flashes of unstyled content on page load, which I thought we'd left behind somewhere in the last millennium.
I had a look around for other people with this problem, and all the examples I found were caused by extentions. Ghostery just fixed theirs a few days ago, for example. https://bugzilla.mozilla.org/show_bug.cgi?id=1404468#c36
Based on the differences among different machines I use with Firefox installed, it is certainly possible that extensions are causing a lot of the degradation since Quantum, but I'm not sure assigning blame is interesting or useful here. The fact is that my experience as a Firefox user is now much worse than it was before. The new extension model was supposed to make things faster, more secure, more reliable, but sadly the result seems to have been very much the opposite.
The new extension model makes better extensions possible, but it isn't going to make good extensions appear out of thin air. It's not going to stop a buggy extension from causing FOUCs or slowing down the rendering of a page.
I don't want to make excuses if you're having a bad experience though. But I'm hopeful about the future of Firefox.
