Do the towers have something like infinite research for range extension similar to artillery?
Just checking, because if not, will need to ignore that research for artillery and be careful with the placement of the artillery and towers. That, or I suppose just occasionally go to look for stuff on the ground or turn off expansions.

Either way, just checking which it is, and making a suggestion if that isn't the case. If that would lag due to the large area, you could try something similar to radars where it doesn't cover the entire area at once, assuming that would help.