Thanks for extensive feedback! ^^ Great to hear that level design does its job, as I'm not that experienced on the matter, hehe.
I'll try to smoothen out some of the trickier jump sections, especially those in the first bird room for the next release 0.1.e. And I guess I should also work on the tilemaps, too! Which tile was the one you thought was part of the background?
And yup, it's is tricky to deliver a steady 60fps on browser, and for optimization I can only do so much (but I'll try to work on that, too.)