How do I... improve performance (especially on mobile)?

Howdy! Thanks for taking the time to take a peek at my question. I've recently released a demo of my game for play-testing, and have had a bunch of playthroughs in the last week or so. The feedback I'm getting all says the same thing: lag is terrible.

This is my first game, and I'm sure my code is sloppy.

You can see for yourself the game here.

I'd be happy to share the .c3p file, but it's 54mb, and I don't really know where to host it. What do people usually do to share this?

See Performance Tips in the manual for some general advice. For future reference it's usually a lot easier to deal with if you identify such problems during development, rather than just at the end.

You should be able to host a large .c3p file on a free cloud storage service like Google Drive, OneDrive, or Dropbox.