1. A computer program product embodied in a non-transitory computer readable medium that, when executing on one or more computers, reduces computer file system access time associated with on-access scanning in a computing facility by performing the steps of: receiving file access information describing a pattern of access of each of a plurality of computer files including a first computer file and a second computer file that have been accessed in a computer file system; processing the file access information to generate a file access performance cost statistic for each of the plurality of computer files; maintaining the file access performance cost statistic for each of the plurality of files accessed by the computing facility; generating a file access performance cost mapping of the computing facility's file system relating to the plurality of computer files; generating a locality statistic from the performance cost map of a probability that the second computer file will be accessed near the time that the first computer file is accessed; and pre-scanning the second computer file when the probability is high that the second computer file will be accessed after the first computer file has been accessed.