This question comes up couple times a week now. Please try to do a proper search before creating a thread.
Are you running the game as a app, or in a browser?
If you are running directly in a browser, you should look at exporting the game as a app instead since that is >100 times faster.Export with Appmobi (Currently only for iPhone/iPad)Export with Cocoon JS (Currently only for Android)
In the near future, both of these options will be able to export to both Iphone/iPad and android.
Then you can also read this tutorial:Performance tips for mobile phones
If these doesn't help, you need to give us more information, because it's nearly impossible to know why without any details.
Do you handle many objects? Do you use any 3rd party plugins? Do you use a lot of particles/physics? etc etc.
And a link/example file would be of help as well.CodeMasterMike2012-07-19 12:32:32