Oh I'm a Firefox user. Also it's not the whole game lagging, just specifically the character. When I set the sensitivity to max, the character moves significantly faster than my mouse but still somehow feels unresponsive like it's a frame behind my inputs or something. It's hard to pinpoint but it feels off to the point where I find it causing difficulties with keeping up with the action.
And yeah it was very enjoyable otherwise, I really like your game design :)