Posting this info without a log is extremely useless. Also I'd be willing to bet this has something to do with something using up your processor or a
problem with a Custom Class. I'm running 5 copies and its taking less than a quarter of a second to cast spells. Which is EXTREMELY fast. No Lag at all. Also try holding your mouse over the little computer icon for the menu in WoW at bottom. See
what your Latency, and FPS both read. Latency being high or frames per second being low can both cause these issues.
Basically post a log, or no one has any chance what so ever at telling you exactly whats wrong. Help us. Help You!