The way the file system in this game works is by using a virtual file system mapped onto the real file system. Basically i means that if you have a long install path name, every resource loaded will have such a long path name attached. This uses more memory but also makes load time a bit slower since the long strings still have to be processed. In order to minimize this i installed stalker to C:S
Also, once you have installed it, i would suggest you run Contig (from www.sysinternals.com) on the dir and all subdirs. It will defragment just those files (not your entire drive)
This can also greatly speed up load times.