A friend of mine said it maybe network-related, there could be so many possibilities. As best as I can describe when I experience lag, is when I look towards a building or any structure. I have red that the dev team have identified a problem with geometry of some buildings, hopefully we will hear more news on this.

I have looked at my CPU, GPU, RAM and VRAM utilization levels on the 'low' graphics preset when walking next to buildings and my results are:

CPU: 50-60%
GPU: 20-30%
RAM: 45%
VRAM: 25%

My computer specifications are below:
CPU: Intel Core2Quad Q9400 @2.66GHz
GPU: R9 270x @1.1GHz
RAM: 6GB DDR2 @800MHz (Dual channel)
VRAM: 2GB GDDR5 @1.4GHz

I hope this may help, if not please ask us as a community how we can help :)