If you spawn so much units for the same player, the units will begin to auto-stoping.
I sugest you making one computer player to control each monster type, and reduce the number of monsters and on other hand, increase their hit points.
Check out my Doom 3 map if you are needing ideas.
Cool usuage of models.
But it has to be greatly improved!
Way too bad, how come a map of a game that is more than 10 years old (I mean, warcraft 3) could cause so much lag while playing? And I've played maps with way higher unit counts, and they did not even cause as much lag is this one did as well.
You've done something wrong, mapmaker. As ridiculous as it sounds, but you didn't optimize the map as good as it should have been.