I was recently experiencing ridiculous slowdowns on Thunderbird - moving a single e-mail to particular folders (ie ones with lots of stored messages) could take up to two minutes. Tracked it down to McAffee (Networks Associates), and found a fix - disabling the "Decode MIME encoded files" in the on-access scanner.
As MIME types are still decoded in the "on delivery" scanner, the risk is minimal, and the positive effects were immediate.
I had been thinking that I'd have to ditch Thunderbird and find a new e-mail client, but not any more.
You could also exclude certain drives/directories from the on-access scan - obviously not removable drives, but certain areas of fixed disks shouldn't be too much risk.
