Start Thunderbird in Troubleshoot Mode. The following article explains how to do that and why it can be useful to help identify the cause of a problem:

https://support.mozilla.org/kb/troubleshoot-mode-thunderbird

Start Windows in Safe Mode with Networking mode. You may have to connect your computer to the router with an Ethernet cable, WiFi is disabled in Safe Mode:

https://support.microsoft.com/help/12376/windows-10-start-your-pc-in-safe-mode

Does any of that make the problem go away? This is how you may know whether the antivirus could be the cause…

Reporter,

Please visit the website to view previous responses to your question and update the ticket.

Unfortunately you (and I) may not have seen prior posts/responses in the past few weeks because the notification system was broken.

Thanks

Hi there Thank you for your suggestions. I have worked out what is causing the slowdown and how to stop the problem though it's not ideal. Bitdefender has an anti-spam filter, which is very useful, once you tell it that an item is spam, it will block all new messages from that address and look for similar messages... However, it doing so, it slows the email system down a lot... So the solution is to turn of the anti-spam part of bitdefender.