Go to settings and use "expert mode" and "save". There is a bug in settings that may slow down your pc. Using expert mode you sholdnt have this problem. In the current sprin version it seems to be the best to disable the Ground decals. Go to "Render details" and set the slider "Ground Decals" to zero.

If it doesnt help: check your system-specification. To run bigger games fine you should have something around a 3200+ Athlon CPU or 3GHZ P4 and 1 GB RAM. Its also important that there are no othe programs running which are using much internet-traffic. Shut them down when your playing spring.