first thing coming to mind is Addons (besides cloggedup windows install -> ie. reinstall windows usually speeds most processes up again)

You may not have updated all your addons to 3.3, or some of your addons authors havent offered a new upgrade and the latest addon is still the 3.2 version.
Also some of the new addons (like gearscore and such) are quite a memory hog in terms of storing every bit of data they see (it remembers characters youve seen to view later) and these addons also communicate trough ingame chat to others on the server or guild with the same addon. Running these addons on all 5 accounts would mean a lot of background data getting pushed.

If this is not the case its either your server getting laggy (on my server the "nr. 1 guild of the world" Ensidia transfered to us including all the fanboi's.... since than dalaran has gotten twice as bad)
or its a problem with your pc, hardware (bad ram) or software (reinstall windows for a clean start again, remember to backup ! )