Similar to the sibling comment, I can't seem to repro. I switched "More Space", restarted Firefox, loaded up a dozen or so tabs. I even watched a YouTube video for about 10 minutes. The video did cause CPU to jump a bit, but it dropped when I unloaded the tab, and nothing I've done (including the video) has caused any sort of lag like described in your first post or subsequent follow-up.

I'm not sure what to say. I'm sure the issue is real, but it seems like we're missing something in the triage. There has to be something else that makes a difference.