I had this same issue. I think the problem is the sheer number of droids in the Industrial Zone. I used a warp band and went around destroying all non-essential droids in the entire zone. With most of the droids gone it didn't lag nearly as much. Although I still had to keep it on the lowest graphics setting.
I also get lag on the plains of Dantooine in both KotOR and TSL whenever I first enter a new module, but then I just refresh my graphics and it works fine after that. This doesn't work for M4-78's Industrial Zone however.
Unfortunately I don't think there's anyway to fix it for me. My computer is just too old :/