Tangentially related, I've seen that behaviour with Google Meet as well. Sharing a specific tab takes a much bigger performance hit than sharing the whole screen.
Tab sharing has code deep into the Blink rendering engine... To the extent that it's actually possible to share a specific <div> or other HTML element, even if it isn't visible! (Not sure if you can do that from javascript, but you can totally do it from C++)
The side effects seems to be that a bunch of the code that prevents the same thing being re-rendered with every frame if it hasn't changed gets bypassed, and I'd bet that kills performance.
