Having a souped up killer ninja computer will definetly help your game's performance, but the sad fact is that the SHIII just isn't very optimized or stable (performance wise) to begin with.
I recently bought a new laptop, a Sony VAIO VGN-SZ360P, and I have seen a few odd ocations where playing the game can go from 100 fps to suddenly very low. I particularly remember an instance where zooming in to the max on the nav-map droped the fps count to around 15!!! ... on the Nav-Map
...oh, and that was without any convoys in view
. Saved and quit, then reloaded and the problem was gone, and I'm like: what the hell?
Let's hope SHIV has a leaner, more efficient and ROBUST programing in it
