Frame rate plummets in large environments

My framerate plummets to 20-30fps once I reach the city area in the first mission, I have an I5 6700K and a GTX 970 do I doubt it has anything to do with my CPU or GPU? I suspect that the culprit is my hard drive as it’s not that fast, however load times haven’t been an issue. I’ve tried all of the suggestions on the steam discussions forum with no success. I have no issues when running Eduke32 or other build engine games.

I’m interested to hear if anyone has similar problems? Has anyone been able to address them? And whether the completed product will be more optimised than the current build.