Hi!

I know that there are many results if I search for "dovecot thunderbird very slow" on Google but none of them helped me with my problem. :(

I am running Ubuntu 18.04 (4GB RAM, 40GB HDD with EXT4) with the following software:

- Dovecot 2.2.33
---- All accounts are using Maildir
---- Sieve filters

- Postfix 3.3.0
---- with opendkim, spamassassin, razor, pyzor, ....

And as IMAP client I am using Thunderbird 68.4 on Windows10.
Sieve is used to sort incoming mails into my 224 folders (56 INBOX. folders and its subfolders (output of "find .INBOX.* -maxdepth 4 -type d -print |wc -l")).

My problem is that it takes very very long to synchonize the INBOX and its subfolders. Even if there are only a few new messages to be downloaded.
Sometimes Thunderbird outputs something like "downloading message 1 of 19" and it takes at least 60 seconds to download all of them. None of these 19 mails is larger than 5kB. And sometimes it seems that the transfer is suspended for a long time and it takes >5 Minutes to synchronize only a few mails. :(

Every helpful advice is welcome.
I just don't know where to look for the cause right now, but I can provide configurations (like output of doveadm config) and some logfiles.

Thank you very much!

Thorsten