Even Teams video calls work fine. A lot of times it works better than the Windows client I use at work.
On old hardware (~10 year old laptop with a Core i5-5200u and integrated graphics), I do have trouble with Google Video calls lagging. This seems to be caused by a combination of old hardware and certain Google products being overly optimized for Chrome.
My guess is that you don't have hardware acceleration enabled. That could be due to Flatpak packaging, or it could be due to running a less-than-optimal graphics driver. Granted it's been awhile since I've tried the open source graphics drivers and I hear they have improved, but I've had better success with closed source graphics drivers.
check if you have hardware acceleration, maybe other stuff? `about:support` has lots of info, I've had to tweak a few things routinely to get browsers working like their native counterpart.