Thanks for playing it! It's not always possible to get a perfect score. Due to how the levels are procedurally generated and the difficulty is increased by having fewer tiles on each level, it's almost guranteed that you'll run in to an impossible level eventually (usually >10 levels in). I'll admit that this could be made better, but the time constraints of the jam prevented me from building a better generator in time.
The slowdown you describe is surprising - This game runs fine even on my fairly slow handheld RG351 and I've made no changes to the game for when it runs in full-screen, so this is very likely related to the TIC-80 HTML-export, which I have very little understanding of. Are you experiencing the same issues in other TIC-80 titles? If you don't mind sharing what OS/browser combo you're using then I'll try to replicate the issue and see what I can do.
I hope you liked the game despite the issues, and thanks for the feedback!