I have just upgraded to thunderbird 3 and have found it to be very sluggish in all respects.
I am running it on a Corsair 16gb Flash Voyager GT drive which is about as fast as a usb flash drive comes.
I have found that switching off Global Search & Indexer fixes the issue, and i think even makes it faster than tbird 2!
Only thing is that it also switches off one of the fantastic new features of Tbird 3
Has anyone else come up with a better solution? My mail folder is approx 1gb.