Glitchiness and Major Slow Down

Here is a link to our CAPX to show we actually have tried with this issue. The enemies used to use tile by tile movement the player has still, which was faster than this, but we're transitioning to pixel by pixel.

dropbox.com/s/q996hnxmcp4ccan/game.capx

We had an experimental CAPX where we were able to have 800 enemies on screen without slow down, complete with dynamic shadows for each enemy... but for some reason, even the enemies being on screen with no code is a major slow down. Go directly downward to witness the 250 enemies slowing the game down a huge amount. This happens to me where it drops to around 7 FPS and I have a quad core computer with 8GB of RAM and a decent video card, so my computer is not the issue, especially since we had that experiment with 800 enemies. I left all the event sheets open in the CAPX that I believe are the problem. Gnashers are the enemy that is slowing it down badly.

Also the Gnashers are extremely glitchy in their movement, and once they are on screen, the game never recovers from the FPS drop even when they're all off screen. How do we fix that as well?

Have you used the debugger on your project to see what is causing the slowdown?

I can't open your project, for it uses all kinds of plug-ins I don't have.

Oh sorry, I forgot to remove the plugins so it can be opened. I tried doing that, if it still gives you the same error, which plugins is it saying are being used?

The debugger says the player controls group and some other groups are using up a lot of the CPU, but it doesn't specify which part of them are using the CPU.

EDIT: It appears the engine is using up 72~% of the CPU.

There's an insane amount of collision-checks going on (sometimes 50.000 every tick) , seems to me that's at least one of the issues.

Well I added for it to only check collisions when on step and that improved things back to 60FPS. Now it seems the game is slowing down immensely from the set custom movement speed action no matter what I do once that mass of enemies come on screen because of all the collision checks I take it.

What's an effective way to reduce collision checks?