Well..there is a solution to solve this problem??Probably pathfinding, but your CPU also seems too old too, I mean... it's still Pentium. Some challenges and co-op (Mind Over Matter being the worst offender) is very taxing in performance. Same can be said to SPs. Firewalking only ran in 20 out if 30 FPS for me.
Next patch maybe???
Graphics of the game don't need higher CPU as RA2 or Yuri's revenge