What anti virus program are you using on Windows 10? Do you have exceptions in place to exclude the Thunderbird profile folders from anti virus scanning. It is common to see the program almost brought to it's knees due to file contention issues with anti virus product scanning every file change. Have you excluded your profile folder from windows search indexing? Another cause of contention that is not helpful at all. I recommend turning off the option in Thunderbird to allow windows search to index mail. It saves a lot of space and removes another function accessing the Thunderbird profile folders. The normal advice when this type of thing occurs it as follows. *Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender. *Restart the operating system in '''[http://en.wikipedia.org/wiki/Safe_mode safe mode with Networking]'''. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: [https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode Windows 10], [http://windows.microsoft.com/en-us/windows-8/windows-startup-settings-safe-mode Windows 8], [http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-7 Windows 7], [http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-vista Windows Vista], [http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx?mfr=true" Windows XP], [http://support.apple.com/kb/ht1564 OSX] ; If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Matt, Thanks for your detailed help suggestions. Here is what I;'ve done. * I'm using Malwarebytes Pro. * I've excluded the profile folder from search in Malwarebytes * I can't find the option to allow windows search to index mail * I started with add-ons turned off and all delays are still there. * I started in safe mode but the long delay on delete is still there-. There is now no delay in displaying a mail's content when I click on it. I'll see what happens if I turn off my AV program and stop background downloads but there is no unusual activity in the Task Manager indicating anything other than normal ops.