Short description of error
Currently blender saves bvh cache files - generated when Cycles has Cache BVH enabled - to a folder called cache that is located at the same location as the config folder containing startup.blend.
Ideally blender should save temporary cache files to the location defined by the environment variable $TMP or it's default equivalent. For a saved blend file, creating the cache folder to be located in the same folder as the blend file could also be acceptable.
An option to specify the location could be an improvement, either in the Performance panel or within the File section of User Preferences. Another possibility is to define another environment variable that the user can set to specify the cache location.