No issues browsing or playing media on Mint XFCE, but I'm not using Flatpak.

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.