What is your anti-virus software?

Thanks for the quick response. I've been using Microsoft's Security Essential for some time now, and most certainly since before this problem occurred. And I regularly Compact all the folders, but that doesn't seem to have any effect.

Try to start *Windows* in safe mode with networking enabled.

- Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode

- Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

- XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still in Windows safe mode, start Thunderbird in safe mode.

- https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

This was an interesting exercise in working through the intricacies of Windows 7.

Then I stumbled on the fact that Thunderbird can be started in its own Safe Mode, independent of Windows7 by holding down the Shift key while loading the program. While in that mode, I told it to deactivate all the many Add-ons, thinking that might be helpful. However, the slow delete problem remained. Then I attempted to start the computer in Windows Safe Mode. However, pressing F8 to invoke Safe Mode start doesn't work. I discovered that others have had the same problem, and it's possible to select Safe Mode while already in Windows7. All you have to do is run MSCONFIG and select 'Safe Boot' under the Boot tab. I did that and restarted the computer, but the problem with slow deleting was still there. So I went back to MSCONFIG and unchecked Safe Boot and re-started in regular mode. The weird thing is that now the problem has gone, and deletions are instant, as they always were before.

Pretty odd. But weird stuff like that does happen sometimes - the problem magically goes away.

http://blogs.msdn.com/b/b8/archive/2012/05/22/designing-for-pcs-that-boot-faster-than-ever-before.aspx discusses some of the features of win8 related to safe mode