I would assume, since there is no data that the calculation (of local Report variables) will be really fast on an empty data set ;-)
I have always stayed away from a Web document with more than 20+ local report variables, again push the logic down to the database instead (=best practice).
You might want to make sure the Webi document is completely PURGED of any data.
See also here for some WebIntelligence Performance tips: http://www.forumtopics.com/busobj/viewtopic.php?p=147179#147179