I do agree that this lag seem to be related to proximity of other players . I was fine when I was playing on my own, but when I've joined group of my friends after about 10 minutes we all started having a massive frame drop to about 10FPS. We've tried to spread around/different servers, but this didn't fix the problem and at the moment game is unplayable for us.

My PC:

Windows 7 Proffessional x64
Intel i7 2600K
Geforce 770 GTX
8GB RAM