Try to avoid the usage of the trigger event and trigger_range.
Those range checks have to be done all the time and for many entities it's many checks each frame.

If you don't need it, turn off GLIDE for c_move. It's slow.

Depending on what kind of level you're aiming for try to block the view of the player to allow better clipping. It makes sense to use BSP then (pro only).
Probably not suitable for outdoor levels.

Instead of scaling down textures on blocks, scale up everything else to get the same result. It's faster.

Remove and purge unused stuff.