Ah, so that's unit control that prevent their vision?
You mean it was squad based in your mod, so easy on CPU? Perhaps you can then do a trick, which is to only have at most 10 units per UPS (or whatever number is reasonable) have vision? I don't need super instant vision, but not knowing at all what is happening is an issue.
The lazy way is to pick 10 units at random. Even that would be a good first implementation...