I've been having trouble with my 3d rts game based on the Metroid universe.

After I have more then 70 units on the screen, the game starts lagging

Samus model is only 80 polligons. And a federation ship is only 60 polligons.
A federation trooper only uses 50 polligons.

The enviroment uses maximum 800-900 polligons.

Does anyone knows what could be the problem?