Tip for placed units, if you find it easier to do so (it is
, unless of course your talking Evolution scale, where its constant).
Waypoints that are looped (ie patrol) should be hooked on a presence trigger, too much moving at anyone time causes the greatest amount of lag.
The most common "laggers" are those in the air, as the x,y,z position is being updated greatly, with no resolve (ie a finish), and its difference is great, hence having a lot can create a lot of lag as it substitutes to even out the distribution of network bandwidth.
Otherwise, make sure spawned units are setup correctly, and try as hard as possible to find scripts with infinite loops, to rid of them as they can create a memory leak, or atleast act as a permanent lag.