Because accessing any file in a directory containing 100+ files in a folder 8 levels deep C:\Documents and Settings\*user*\Application Data\Mozilla\Firefox\Profiles\default. \Cache\ is going to be slow, moving the cache folder to C:\Cache would make it a lot faster.
- navigate to about:cache to check out your current cache usage and directory
- navigate to about:config
- create a new string browser.cache.disk.parent_directory
- a value of C:\ (preferably a separate partition) would put the cache directory at closest to the root