There can be all sorts of reasons that specific sites or specific installations might show performance problems that aren't representative of the typical experience. Your specific environment might have some feature (particular hardware, profile data, addons, etc.) that happen to trigger a bad case in some code and so lead to an overall slowdown. Or you might be regularly using a site that happens to do something that's CPU intensive in Firefox but less so in other browsers. This could be a Firefox problem (maybe we implemented a feature using an algorithm with different tradeoffs compared to other browsers and it so happens that everyone avoids the worst case behaviour in Chrome, but not in Firefox, maybe our implementation just has issues that can be fixed), or it could be a site problem (sometimes sites just send buggy, broken code to specific browsers that causes them to use lots of CPU for no reason).
One of the things that was most valuable during the Quantum project was the work to improve the profiling tools. If you can capture a profile using the super-easy-to-use gecko profiler[1] and create a bug with the profile attached it should be possible to figure out what exactly is causing the problem in your case. Without that data it's really hard to make progress because the kind of problem you experience is just not something that would be allowed to ship if it was known.
[1] https://developer.mozilla.org/en-US/docs/Mozilla/Performance...
It is a showstopper. I don't care about new features if it can't handle basic stability.
Oh, and before anyone asks, on the thinkpad the performance issue was still present on a debian stretch install as well.
Just because it works on your system doesn't mean this isn't a bug they need to fix.
If it was broken for everyone it would already have been fixed.