helena - sounds like you've walked right in to the mistake outlined in that blog post Arima linked to (Remember not to waste your memory). A long epic background might be really cool, but mobile devices often don't have much memory and you have to be smart about how you use it. If your images are over 2048x2048 that can also severely impact performance on some devices, and missing images are probably out-of-memory errors. There are good ways and bad ways to design mobile games; desktop PCs can generally muscle through anything at all, but mobile devices can't, so you need to be careful how you design your games and test regularly on mobiles from the very start. If you share your .capx I could comment in more detail about what might be slowing it down.
stevefromio - if you're so convinced that HTML5 sucks, perhaps you would like to select a different tool for game creation, and post in their forums instead.