I had a few minutes to play with this early beta. For starters, great work - all emulators I've tried seem to work, and my controllers connected without any issues (correctly button mapped too!).
It is clearly a lot of optimization still needs to go into the build for optimal performance - which is mostly apparent in N64 (mk2) and PSP (ppsspp) - but I am not the correct person to advise on this. One thing I will say, however, is that I think libretro glupen would perform better in most cases than mk2.
Here is small thing you may wish to include in extlinux.conf: video=HDMI-A-1:1920x1080@60
Unless you set the resolution in extlinux, it will automatically be 4K for all emulators on higher DPI displays, which kills the performance. There are very few people who run at a lower res than 1080p anyways, so this is a simple and good fix.
I'll keep on testing in the coming days and come back with more feedback.