I got an idea... Lag experts look here.

...and don't hurt me too much if I'm completely wrong. I've only got my own experience to speak of when it comes to lag (not necessarily connection lag, but general graphic/control choppiness online -and- off). I've gotten this mostly on large maps with many preplaced units (and likely doodads), or sometimes even on small-medium sized maps with many units moving around at once (this even in single player, in fact for some reason I lag more in single player than in an identical map on Bnet, answer that ;)).

Anyway, I've thought about creating units as they are needed instead of pre-placing them. Actually, only creating units when the player(s) controlled unit(s) are within a certain range of an event/area that the units are supposed to be. Likely, they would be destroyed in a similar fashion once they had served their purpose - if there is no mechanic of the campaign that might make you run into them again.

So that's my idea. Lots of triggerwork but less lag. Would it work? Pros/Cons? Or am I just completely stupid :D

It might well lag more if you were continually creating and destroying units.

Are you sure that it would though? I think it would be quite a bit of work to test this out just to find out it would lag more... There are ways to remove a unit from the game + memory isn't there? Like exploding, instead of removing? Setting everything to "Can't raise, doesn't decay?". I'm aware of the lag caused by creating large numbers of units at once, but my idea was to not do that exactly but create them in small numbers only as needed.