If I were you, I would revert to the previous version of Windows first. You don't have to upgrade to 2004 just yet. Give it a few months for issues and bugs to be addressed first.

Thanks Stans but the deed is done. I hope that if anyone else has seen this since the 2004 update that the developers can look into it. I am not sure it is a Windows problem as I "think" it may have started before that Thanks for the reply!

I don't understand. Are you saying you cannot revert to the previous Windows version, or have you already done so?

What are the security apps on the computer? If you have them acting on the TB profile folder, it's possible the AV app became incompatible with the Windows update, and resulted in degraded TB performance. Try running in Windows safe mode to test if startup apps are causing issues, and (always) exclude the TB profile folder from AV scans - even if the AV is Windows Defender, but especially if it's Norton, Bitdefender, Kaspersky etc.

It has been more than 10 days since the feature update installed so no easy way backwards I use Windows Defender and firewall. The profiles folder is excluded from scanning and I have in and ou rules so that Thunderbird is wide open I rebooted into safe mode and there was little improvement if any. I then put Thunderbird in it's safe mode and there was a bit of an improvement ... it takes about 5 seconds for a new message window to appear

In the middle of all this, KB4557957 was pushed out to me and that seemed to help getting a window in 5 seconds with a normal startup

IT isn't right and I am not sure it is a windows issue Thanks

I doubt the Windows update is an issue, and since the problem exists in new profile with no add-ons other than Lightning, I wonder if it's account-specific, i.e. syncing or indexing of very large IMAP folders or calendars is causing a slowdown. You could go through the list here and see if any of it applies to your situation.

Thanks for the list. I think I will uninstall and reinstall it and see what happens

Some systems experienced slowdowns from the April cumulative update which Microsoft is yet to acknowledge two months later. The issue persists with the May and June cumulative updates and feature update 2004. The only solution to it was and still is removing the offending update. I'm one of the affected users, so I haven't applied any cumulative updates since the March one to keep my system running optimally. I noticed the slowdown when creating routine system image backups using Macrium Reflect, something that used to take about nine minutes to complete, running at average 700mbps suddenly took four hours at a crawling 40kbps average speed, when the source drive is an SSD and the target is a USB 3.0 drive. The problem is yet to be acknowledged by Microsoft nor resolved in current updates. Let us know how it goes after reinstalling Thunderbird.

Thanks Stans. Reinstalling didn't do anything for that issue. I took it out with Revo and then removed any applicable folders etc I even set the priority class to "high" as well as the i/o priority none of which did much

@sfhowes gave a link to troboubleshooting and I have been going through that if for no other reason than to rule out a Thunderbird issue

I think you may be right about it being the 2004 update. I think it is going to give developers nightmares before it is over. I beta test a graphics editor and one of my favorite built in shortcuts stopped working. Beta issue? Not quite as it stopped working in the commercial release which was early this year where I know it was working Alpha testing for Miscro$oft is just grand :))