What is your anti-virus software?

Do you get any 'Not responding' notification in the title bar?

What is the size of your Inbox?

Are you subscribed to the 'All Mail' folder?

Thanks for the prompt response.

I use Malwarebytes and Microsoft Defender. I turned off both, but the same lag was still occuring.

Note that sometimes there is only a small lag (1-2 seconds when clicking on a mail heading for ex.). Most of the time there is a 3-7 seconds lag however, for every action, including in the calendar tab.

There is never a "not responding" notification, nor does it all go white-ish, like it would do when freezing.

Gmail inbox folder: 1881 emails, 183MB. Gmail All Mail folder: 6950 emails, 1200MB Murena inbox folder: 4445 emails, 475MB. Check out attached screenshot for more details.

I can't find an "All mail folder" to (un)subscribe to. There is an All Mail folder which is part of Gmail, and also a "unified folders" option through the Thunderbird email menu, which I don't use.

Thanks in advance for any help. I am missing a major tool in my daily workflow!

I use Malwarebytes and Microsoft Defender. I turned off both, but the same lag was still occuring.

The only reliable way to turn off antivirus Software is to start Windows in safe mode with networking enabled. You may want to try that.

Does the problem go away?

Indeed, in safe mode with networking, TB is lightning-fast. (I wasn't able to connect with the Internet though, not sure why that is.) As soon as I reboot into normal Windows 11, there is a short lag for each action, and later it can get much worse.

Does it mean that either Defender or Malwarebytes is the culprit? I could uninstall Malwarebytes, but if I can't turn off Microsoft Defender aside from going into safe mode, what can I do?

...and a correction: I just had TB freeze for about 15 seconds with the "not responding" notification, when I deleted about 30-40 selected emails. After that it was back to "normal", meaning just about useable, but slow.

Does it mean that either Defender or Malwarebytes is the culprit?

Most likely, yes. Create an exception (or exclusion in case of Defender) in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

I could uninstall Malwarebytes

Sounds like a good idea to me. Running two different anti-virus scanners at the same time is a bad idea anyway.

if I can't turn off Microsoft Defender aside from going into safe mode, what can I do?

Create an exclusion for the Thunderbird profile - see above.

Okay I created an exclusion of the profile folders in Defender and restarted the laptop. Afterwards TB was quick, and for the first time in a long time I was able to sift through many emails without a lag for each action. But after about an hour it froze for a little while as it started downloading a huge number of messages that I had deleted a few minutes before. After that there was again a moderate lag most of the time.

Now I am getting an error I haven't seen in a while. In fact, I remember getting that error long ago, when there was no lag issue at all... So just in case here's a screenshot.

Now I am getting an error I haven't seen in a while.

For which account?

But after about an hour it froze for a little while as it started downloading a huge number of messages that I had deleted a few minutes before.

Also, in which account did this happen?

The network and file access error is for the e.email account, from Murena.io, a privacy-focused, Nextcloud-based alternative to Google. It happened again this morning but is not very disruptive.

The very large number of messages I deleted were from my Gmail account. I tried a second time (permanent delete), but they were downloaded again later on.

These 2 issues are not what is bothering me, but they might indicate where the problem lies.

Since I installed a fresh version of TB with a new empty profile, it might be one of those two accounts causing the lag issue... or not...