What is your anti-virus s/w?

My anti virus is Bitdefender but the problem is much mere recent than the installation of Bitdefender and my inbox is the only folder affected

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?

I tried this and, in safe mode it worked properly. I returned to normal mode and it was slow again. I disabled all extensions except British english dictionary and there was an improvement. I can't manage without my calendar, so I enabled Lightning and slow again. I've had Lightning for ages so it's odd to imagine it causing the problem. I opened the calendar in Lightning and high speed was re-established, closed the calendar - slow again ! I moved a few files about as a test - fast again - then slow again - then fast again. It has settled down fast, so fingers crossed and hope ! Thank you Christ1

It's still working 2 days later, although I have no idea what was wrong and, in particular, how only 1 of 4 inboxes in my email system was affected. Thanks again for the help

The problem has started again and this time the fix has not worked. If I uninstall Thunderbird to install a completely new version, will I lose my stored emails and email settings ?

Exclude the Thunderbird profile folder from being scanned by your anti-virus real-time scanner.
Typically you'll need to create an exception for the profile folder. Check this article for help to locate your profile.

http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Thank you for that info, which I have bookmarked for future reference. I detached 3 photographs, which were present when it was working properly and it started working properly again, so fingers crossed and hope it keeps working.

Deleted - accidentally posted thrice

Deleted - accidentally posted thrice

If you think your problem has been resolved, please mark this topic as 'Solved'.
Thank you.

I have the same problem with very very slow deletes. I think it started with the last Thunderbird update.. I found the profile file but I have no idea what to do then, ???