1. I have a large inbox in Gmail and assume that this causes the performance issues that I am experiencing in Thunderbird (program freezing for a couple seconds once in a while).

The root cause for the freezing is not necessarily the size of the Inbox, but more that anti-virus software takes longer to scan larger files than smaller files.

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

I understand, the synchronization settings, in which I can choose to synchronize newer emails only, affect only whether the body of the email is downloaded but does not decrease the size of the data base of headers.

I think compared to the size of the email bodies, the size of the headers can be neglected.

I need these emails to be stored on-line and, in fact, do not even want their bodies occupy space on my disk.

Make sure not to turn on synchronization for the folder "archive" in Thunderbird. https://support.mozilla.org/kb/imap-synchronization

I want this folder to synchronize only when I open it. The emails in this folder should not appear in my inbox

When you archive a Gmail message from the Inbox, the message can still be found in All Mail. See this article for more information on Gmail particularities. https://support.mozilla.org/kb/thunderbird-and-gmail

About the Thunderbird Archive feature. https://support.mozilla.org/kb/archived-messages

Thanks for the early reply (and sorry for the late response). I should have added a few more information:

• The freezing does not occur so frequently that debugging is easy. Sometimes everything works fine for a day. At other times, Thunderbird hangs and actually does not respond at all several times in a short time interval.
• I have taken some measures to reduce the imact of the OS on the performance. For example, I have disabled folder indexing in the profile folder. I am not using any third party anti-virus software - just the MSSE built into Win10.

I cannot run Windows in safe mode until the problem reoccurs, because the problem is too rare and I need to use the computer. However, I will try using Thunderbird in safe mode for a while and get back to you with the results.

Until then, I have a couple of questions, still:

1. What causes the high disc usage? If gigabytes can be freed frequently, then these gigabytes must have been in use before. What process is responsible for this? Could I stop that?
2. Can I exclude the emails in the archive folder from my inbox so that I am not including these emails in my searches? I want to continue using the inbox, as the emails from all my other accounts are there, too.

On another thread, the user has to create an exception in Windows defender to allow the global index to generate. So the basic Microsoft tools are not except from cause issues.

It might be the compact is not completing, so your seeing the same Gb over and over. Again. Anti virus is a common issue.

Gmail has an all mail folder that holds all mail. It is the archive. Why not use it? that is remove the mail from your inbox instead of trying to manage it as an archive, which it is not.

Thanks for your help!

Matt said

On another thread, the user has to create an exception in Windows defender to allow the global index to generate. So the basic Microsoft tools are not except from cause issues.

This would have to be a very common issue then. I would not like to mess around with my security settings. I will look for this thread, though.

It might be the compact is not completing, so your seeing the same Gb over and over. Again. Anti virus is a common issue.

The GB I am seeing are different from time to time and accumulate over time, if I wait with compacting. Nonetheless, I cannot see the promised space freed on my disc after compacting. So is that a sign that there is an issue?

Gmail has an all mail folder that holds all mail. It is the archive. Why not use it? that is remove the mail from your inbox instead of trying to manage it as an archive, which it is not.

I did not fully understand this. Isn't removing the emails from my inbox exactly what I wanted to do (unsuccessfully)?

I am not using any third party anti-virus software - just the MSSE built into Win10.

That doesn't necessarily help. These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Can I exclude the emails in the archive folder from my inbox so that I am not including these emails in my searches?

I don't think I understand what you're trying to achieve.

I want to continue using the inbox, as the emails from all my other accounts are there, too.

Accumulating messages in Inbox isn't best practice, and it does increase the risk of mail file corruption due to anti-virus software interference.