Talking about "the issue" is a misunderstanding of how performance of complex software works. Overall performance is the result of the entire codebase, and there can be a varieety of ways that performance problems can manifest for individual users. One of the most productive approaches to improving performance with Quantum was fixing lots and lots of small and medium sized issues across the product that individually didn't have much impact but overall added up to a significant improvement.
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.
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...