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.
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?