There could be so many causes that no one here will be able to give you a perfect answer with virtually no information about your game. I'm far from an expert when it comes to optimization but:
1. Have you previewed the game in a mobile browser to see if the cause of the lag really is the crosswalk build?
2. Have you tried more than one mobile device?
3. Have you looked at the debugger to see what part of your project is using most of the system resources?