Thre are many things to keep in mind when designing for mobile - the devices themselves are far weaker than desktops, physics is extremely CPU intensive, you need to be mindful of overdraw (pixel fill rates on mobile is lower - I read somewhere that a lot of current devices can only draw 3x the number of pixels of the screen per 60th of a second), semi-transparent pixels take more time to calculate than fully opaque or transparent ones, limiting collision detection to the fewest needed with conditions, rotated sprites aren't as quick to draw as non-rotated sprites, previewing over lan has a lower framerate than exporting the html, etc.
If you implement some of these optimizations, there's a good chance you'll be able to get it up to 60 fps.