I suggest reporting a bug to Bugzilla. The positive side effect will be you will see how efficiently they handle such reports. And you will get it fixed.
I have the same issues on Linux. It's really fast, but often it hogs one core completely and the CPU does a lot of work, sending the fans in overdrive. Especially when videos are involved.
Let's hope things get ironed until a stable release in november
Mention Quantum Flow and "performance" in it. Make sure to attach a performance profile with https://perf-html.io/ using the linked add-on there.
I had many cases where the mozilla devs were glad about such bug reporst (especially if you are on Linux - I am too :) - which is not so often used and reported like Windows or Mac)
I have the same issues on Linux. It's really fast, but often it hogs one core completely and the CPU does a lot of work, sending the fans in overdrive. Especially when videos are involved.
Let's hope things get ironed until a stable release in november