Exactly what shagrat said above. Also, some some effects in game still kill FPS on alot of rigs, such as tracers, flames/smoke(in a bigger scale ofc, not one tank alone) and some other stuff ive seen people stating around here.
Also, if you are running 1024 resolution with every pixel set in options, thst also may kill your fps, especially looking at a large area in your tgp.