Firefox is doing a good job about analysing extensions startup performance, but I guess they need to do the same thing for page loads and memory usage. Or integrate the extension functionality into core more.
Unfortunately in practice what's happened so far is that lots of useful little things aren't possible any more because the new API doesn't support them, while extensions are evidently still capable of causing problems with performance and stability anyway.
Hopefully both of these aspects will improve in time. In particular, the emphasis on personalisation and improving the extensions ecosystem in this roadmap is welcome and seems very much in keeping with what a lot of us used to like about Firefox. I hope they really do put their focus there and exercise caution on becoming too "opinionated".
Just to add another data point, this has also been my experience: Firefox since Quantum often pegs CPU cores at 100% when showing pages with even quite simple animations.
The other big trouble area since Quantum seems to be extensions. After various experiments of my own and various discussions online, it seems clear that the machines I use where there are more extensions involved are much less stable and much more vulnerable to problems generally than those with mostly vanilla Firefox installations.