I have noticed that on my PC my FPS really suffers (averaging about 15ish) when I'm in an area populated with other runners like P1, however is capped at 60 through most other areas and situations. Whereas my laptop, running much poorer spec seems to not be affected at all by this and can sit in P1 50+ FPS.

Are there any known issues why my PC struggles with this when it can run much newer games on high gfx settings with no problems?

Some of my PC spec;
Windows 7
Processor: AMD Athlon(tm) II X2 250 Processor (2 CPUs), ~3.0GHz
Memory: 4096MB RAM
Gfx Card: AMD Radeon HD 7700 Series
Gfx Memory: 2799 MB