Nothing much I can really do about any of the issues as I've tried fixing them already. During editing, when I tried adding decoration, it just lagged it even more. FPS would drop below 10 down to 5, so I was forced to remove them. With the map already being fairly large, I can't really add anything otherwise it'd lag even more to the point of it being unplayable, or at least for my case, uneditable. For the spawning, I honestly don't know anything about the engine to really be able to fix that. The wires that connected to the door were there, but the zombies would still spawn like you mentioned, so I was, sadly, forced to leave that there.

In the starting area, zombies tend to spawn behind the second door before level 7, even if the door only opens on level 7, so you're stuck waiting until the next. The level looks kinda empty and barren as well, due to the lack of decoration. Also, I tend to get framerate drops, slowing down the game. Dunno what you can do about all that (except the decoration maybe), but hope that helps.