Hmm. OK I'm back on my desktop x86-64 with Firefox 74 on Fedora 31, and it runs a lot faster than on the Odroid Armhf64 mini-PC. Subjectively it is about twice as fast. Fedora has a screencast recorder built in; I was able to capture 30 seconds of that. I'll have to see what is available for capture or debug on Arm/Ubuntu.
[Edit] I think that was a waste of time - I need to learn a lot before I can do useful screen capture. But I'll post what I captured. I'll post links first then experiment with that little video icon in a separate post.
alien-x86_64.mp4 I suspect the main problem with this is that the screen capture built in to Fedora probably defaults to a low frame rate. The speed of the video seems similar to the real thing, but the video is more jerky, like it is missing frames.
alien-arm.mp4 I installed something called simplescreenrecorder. I said the Odroid was already slower than the x86 box. When I started the recorder, the browser frame rate dropped to ~3 fps. Underwater molasses. The speed shown in the video is pretty much like what I saw on the screen during the recording. But when I turn the recorder off it is a lot faster than that. So, more research required for sure.