This is a great little game and I've enjoyed playing it quite a lot! But I'm having a bit of an issue with missed inputs and delayed / laggy keyups with both this and another game of yours that I played (Pakpok). This is playing on Firefox 83. This might be just the computer being too slow to run the cart, but I don't remember any issues with PICO8 Celeste and played a couple of levels just now and didn't see any missed inputs.
This hasn't made the games unplayable - I only see missed inputs or delayed keyups every few minutes or so, but it's very annoying when it happens.
I recorded a delayed keyup in Pakpok along with my keyboard inputs, and slowed it by 50% to make the issue more obvious. (The issue looks exactly the same when it happens in Flip Knight.) You can see in the recording that I continue moving to the right at least 14 frames (recorded at 24 fps) after I've released the right arrow key.