How to quickly empty a very full recycle bin?

Do you have a virus scanner that does on-access scanning? Disable it during your delete action.

With my virus scanner disabled deleting is over 10 times faster.

(I never understood why it checks files that are about to be deleted)

There's RecycleNOW, a small utility that empties the Recycle Bin immediately, no matter how much it contains. (Or at least I've seen it run very quickly even with my Recycle Bin containing ~1 GB of data.) Works with Windows 7.

Once you get it, just double-click on the executable file, and it's done. The Recycle Bin should be empty.

It shouldn't be that slow, unless you have shell extensions installed that hook the delete process to do something (like the NTFS Link Shell Extension's "Smart Delete" feature).

But you can always try

RmDir /S "C:\$Recycle.Bin"

and

RmDir /S "C:\RECYCLER"

and they might be faster. They'll delete your entire recycle bin on C:, though (including other users' files).