I have heard from others that they've experienced no issues with the Linux version. Low framerate has been a rare reported issue with all builds, though. I'm not sure what's causing it. Did it seem to get worse the longer the game was running?
Viewing post in The Dark Queen of Mortholme comments
it stayed consistent even through restarts as far as I am aware. on subsequent runs I noticed the queens other attacks also not working just as the hero's melee. i tried using emulation through proton with the same result, so not sure if it is hardware related, though my pc is up to modern specs and this is just a 2d game.