I bought a Raspberry Pi 3 A+ hoping, among other things, to play HD videos. I have flashed Raspbian Buster on a 64GB microSD card, extended the FS, and, generally, the Pi seems to work fine with light stuff (like browsing simple web pages, including YouTube).
If I switch Fake/Full KMS on, however, everything on Buster becomes slow and sluggish. For instance, fully opening Chromium takes about a minute whereas it's a matter of seconds without OpenGL. If I then go and do something (not necessarily heavy) the Pi becomes unresponsive to the point that the only solution is to unplug the power cable (I'm using 5V at 2A btw) as even trying to ssh into the Pi is impossible.
To me, this behaviour looks very similar to a memory starvation issue. I have tried setting
gpu_mem to some higher values than the default (which I think is 64M) but that didn't work. Setting it to 128M makes even loading the OS worse (about 5 minutes to get the full Raspbian desktop). Values of 256M and above cause a
cma: failed to reserve 256M error message to appear in the log.