How to make enemies bang doors

[Your Computer]

If it's in the map, you mean like if it's permanently active and not summoned by something like a scripted area?
I've preloaded all my particle effects and sounds, but I don't notice the slightest difference. Is it only noticable on slow computers? How slow are we talking?

A particle system can cause temporary lag on any computer, specifically those without a solid state drive, but it depends on the particle system itself. A PS file is really small in size, at least compared to the textures it references. If a particle system references a lot of textures, specifically textures that near 1 MB or more, then you may experience lag when it is being loaded.