Hmmmm, it's tough for me to test fps as I only have 1 pc and it's a little on the high end and I only test single player. I believe it has to do with the number of orders being given to the AI at one time. At what points did you notice the most lag? I notice fps drops when there's a lot of units in one place. I might make stairs unwalkable and use invisible platforms but I don't believe that's what's causing it. The functions for the ai are pretty fierce right now, I'll make that my main concern to simplify it for v1.3. I feel there's a lot that can be reworked but it's gonna be a bitch haha.
Edit: Made some major adjustments to the AI to try and smoothen it out, less uruks further apart seemed to help a little, but the major problem was that trolls, banner carriers, wargs, and crossbowmen didnt have a food value, causing them to stack up in count over time. I fixed that and it seemed to run much smoother throughout. Also helped to balance the game out a bit. you now get double points per kill with less uruks too. Anyways let me know if This runs any better please and thank you!