FPS Lag in Command & Conquer Generals : Zero Hour. From The Ultimate Collection

Hello,

I am playing on the skirmish mode against a Medium army and I will get FPS lag that is too big to just deal with. I initially thought it was just the amount of troops/buildings I had, but if I go to the challenge mode and make more than double of what was in the skirmish mode it does not lag in the slightest.
          Thank you