Out of nowhere (i.e., I can't trace the start of this problem to a specific change to WP) I've found that WP-ADMIN loads horribly slow. Like tens-of-minutes slow. This only affects the admin section of my site - not the site itself.
After troubleshooting everything I could think of, I've discovered that the pages only load slowly in IE, FFX and Chrome. But it's blazing fast in Opera.
That's right, Opera!
Now, what makes Opera different from other browsers? Well, I seem to remember that in order to be the "fastest browser around" Opera begins rendering a page BEFORE all elements are loaded. Whereas other browsers don't display anything on a partial load.
I confirmed this by the following:
1. Opera loads the page, but I still get the "working in background" cursor on each admin page.
2. Using Firebug in FFX I was able to see the page load before it was completely loaded (couldn't interact with it though).
3. In IE anf FFX, if I click on a link to my admin page then get up, make some coffee, go to the bathroom, walk around the office for a while then come back to the computer, the page WILL eventually load.
So, it appears that the problem is caused by something trying to load on the page but not succeeding. But whatever it is, it's not causing a timeout error or a 404 error. Ive tried disabling and re-enabling plugins to see if they're the culprit, but no luck. Tried changing the memory allocated to PHP on my server and no luck.