In our environment we have a 6.5 farm running UPM 5.1, over the last couple of years migrated to Internet explorer 11 as part of our standard hosted shared build. One thing took us a little while in getting around to doing as part of housekeeping for this process was to clear off IE8 cookies from users’ UPM profile. When moving to IE 10 (or straight to 11) the cookie infrastructure has changed to a single chunky webcacheV##.dat file.
Now although these files are small you’ll probably find for a typical user they were numerous thus a pain to process on logon (lots of small files BAD vs one biggish file GOOD). Just delete them as useless but UPM may still look to take them down every time you logon thus slowing up your logon needlessly.