Also, FYI, I’m on 0.4 of the OS, but performed the upgrade to Buster as described here: GameShell OS image files (v0.4 latest)
I’m also running launcher “stable 1.25”.
I did my MAME testing with the Lima driver. But I just tried it with FBTurbo, and I’m honestly not sure I can see any difference. So that makes me think this version of MAME is using software rendering only. When I was messing around with this (and on Lima), I tried the various rendering options inside MAME (via built-in menu) and didn’t notice any differences. There are options for Open GL, but I’m pretty sure they didn’t work (for me, at least!)
I also tried to turn off all the “fancy” rendering stuff. MAME does a great job these days of filtering the screen when stretching it, and also making it look more like an old monitor, but all those fancy effects just kill the framerate on a device like the GameShell, so I turned them off. Or at least I think I did. There may still be room for improvement or something I missed, so if you find it, please post the changes here. 
If we could get a custom build that actually uses video acceleration, I’d hope MAME would be able to handle more games at full speed, and maybe even support some of the filtering on older games that would have plenty of juice left to spare.