Which game have you been attempting to run with beetlepsx? So far, I've tested with resident evil 2, dino crisis 2 and mortal kombat trilogy, and they all seem to run at 30FPS most of the time with some dips along the way, but it's been preety consistent so far. Though, if you attempt to mess with the graphical settings, it gets unstable and not playable at all.
Also to note, in my surface pro 4 with i7/16g ram, runs about the same, though, at least I can mess with the graphical features, not too much either.
I also tested mame mith mortal kombat roms 1-3. loads and plays fantastic. I attempted to run mk4, but, as you may have guessed, it doesn't boot on any of the mame cores minus mame 2010, but it's almost power point like and it's broken. On my SP4, it loads on mame2016, but it's unplayable there too, so not to shabby for this version of retroarch on xbox one.