There is a giant confusing tech stack here, but opening up this simple plain text page resulted in like 100% CPU usage from from gnome-shell when visiting from firefox on a pretty non-customized Ubuntu 24 install.

In the firefox task manager nothing really looked odd, but opening that tab and displaying it is insanely CPU intensive.

Pausing the autoplaying video makes it seem like a sane web page in terms of CPU usage. I'm surprised how much CPU playing that video consumed.