Oh darn this really sucks. I didn't get to playtest on a weaker device or optimize. I feel really bad; sorry >_>
I dunno if you already have but maybe try checking to see if hardware acceleration on your browser is on. Without it the game might struggle
But I'm glad you liked the stuff before the boss. I'll try to fix performance after the jam, or just release a non-web version which should improve things by a lot.