Slow imap connection to local dovecot server

[Lothar Paltins]

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?