Rogerx, I have tried countless of missions and combinations of startup parameters. My system specs are as following:
CPU: AMD FX-8150 (overclocked from 3.6 GHz to 4.4 GHz. Also tried disabling 4 cores, one on every module, as suggested by people online, made no difference)
GPU: MSI R9 280X 3G
RAM: 16 GB 1333 MHz DDR3

The game is also running on an Intel 330 SSD.

I've tried custom memory allocators, the allocators supplied with the game, none seem to work. It's just horrible, even in games without AI, my performance is severe. On empty servers, my FPS doesn't get above 35, while CPU and GPU usage hardly ever goes above 50%, and neither does memory usage. There's just nothing that seems to bottleneck because nothing is used fully.

I guess that I just have to deal with the average of 15 FPS I get, while all other games I play can be maxed out, easily.