I don't know where is the problem coming from...
In my PC with an old intel core 2 quad I have the following stats:
Firefox: 58-60 fps, 0-2% cpu.
Chrome: The same as above
IE11: 60-67 fps (!). 4-7% cpu.
Please mind that I also play the game in a 2660-1440 monitor and it is huge in size (since you have set the game to letterbox scale) and even at this resolution I have no problems.
Perhaps something is up with your graphics card? Have you installed the latest drivers?
I read your code and I didn't find any loopholes or something. Everything looks like it won't cause any problems. I think that your event sheet will greatly improve if you use families (include all enemies to one family for example), and some actions could be made calling a function. Also, you could use very small sprites and lessen the download size and since the game's aesthetic is pixel art, it could benefit from point sampling and pixel rounding (at the project settings/parameters). Also, try not to use too many sprites if you can avoid it. You can use the same sprite with an instance variable to check for collisions, also, the life bar system can be replaced with a single tilledBG object. And finally, use actions to create/spawn the enemies, you don't have to use placeholder sprites: See the events 16 and 36 inside the attached capx.