Firstly check out the manual: scirra.com/manual/134/performance-tips
and of course: scirra.com/blog/83/optimisa ... -your-time
If it is a single moment where you are experiencing a lot of "lag" then you might be in luck - start the game in Debug mode and then use the debugging tools to see how many Collision checks, poly checks, are running at the time of your event.
If you can accurately reproduce the performance problem then you know EXACTLY where it happens. Take a copy of your game then try to fix it step-by-step by adjusting things.