The symptom was increasingly long (as in literally 5+ minutes) to get from when I clicked on the application icon to when the login screen showed up. It wasn't network lag, it was application start lag.
It turned out to be a function of how much stuff is in the Interface folder: apparently WoW parses absolutely everything in there, not just its own Addons folder.
I had been duplicating my AddOns folder each time I did an update from wowace, since so often careless authors upload extremely broken mods, and I needed to be able to revert. I assumed WoW knew about its internal folder structure, and would ignore folders that were not its own (at least that's how *I* always design my applications...)
My normal loading times resumed when I deleted everything in "Interface" other than its AddOns folder. For good measure I also removed a bunch of old addons I wasn't using anymore, but I think it was the 17 copies of "AddOns" that were the real issue.
I use a Mac, so I'm not sure if it's platform-specific.
Hehe, yea, basically WoW makes an internal listing of all files that fall under /World of Warcraft/, to create a sandbox. Basically by having an abundance of files (by having 17 backups of your AddOns folder), this listing takes much longer to create.
It really makes an amazing difference, heh. (I also tackled my screenshot directory. And did you notice that WoW Test is under the World of Warcraft folder? I moved that out of the way, I'll move it back later.)
The other day, I had an issue with Threat. Every time a hunter misdirected to me in a non heroic Black Morass, I got an error with something like "Recount/libs/Threat 1.0." I apologize for not having the exact text, but for whatever reason, I am unable to recreate the bug on either my rogue or warrior on world mobs.
When the error window would pop up, it would continue popping up until I left combat. Sorry I don't have more info for you.