We do have such a script. It runs on the log file and can give you the top N (default of 10) longest running rules. It is an unsupported utility, but it is written in Perl and easy enough to modify.
If you navigate to on support net to here : http://supportnet.mentor.com/downloads/related/calibre_utilities/files.cfm
You'll see a list of unsupported utilities; scan down to the bottom for view_log.zip. It is unsupported so you may have to tweak it to get it to work for you.