yeah i get the feeling you are using 5 different wow folders

this causes the HD to do overtime loading every file 5 times trough those tiny little cables and cache memory

easies way to try out if this helps,
load all wow sessions from 1 !! folder (ignore messed up ui and such)
if this makes the lag go away you can try out symbolic linking (depends on if you use XP or Vista, search the forum for it) by 'replicating' the wow/data folder, so each file gets loaded from the same physical spot on the harddrive, less lag