UE4 freezing up on me now

Been working on a 4km x 4km town map and now it’s starting to stutter and freeze for a few seconds. Just wondering what I could do to get back that smooth feel I once had. Was at Eipic settings now set them to Low.

I5 4690k
GTX 970
Windows 10

I would suggest you have a system problem of some description, rather than Unreal Engine being the issue. Outside of that, I’m not sure what to suggest.