Slow imap connection to local dovecot server

Hi,

this is my setup:

OpenSuse tumbleweed dovecot 2.3.0.1 (ffd8a29) thunderbird 52.6.0

I'm running a local dovecot server on my private network to be able to access my saved mails from several computers on the network. The network is behind a firewall, therefore I'm only using plain imap on port 143 with plain text password authentification. But very often the login from thunderbird to the dovecot server takes a very long time, even if thunderbird and dovecot are running on the same machine. It can take more than a minute until I can read a message. I dont't have this problem with the remote imap servers of my mail providers.

Does anybody know, why this happens or how I could debug this issue?