it looks like SOGo logins and logouts for every request to the IMAP server.

IMHO this would require a permanent connection to the IMAP server of sogo.

very likely when this is implemented 0004789 would be fixed, too.

Did i miss a possibility to configure SOGo, so it keeps a connection to the IMAP server for better Performance?

Keeping one connection per logged in user is a resource problem.
For installations with a couple of users that would be possible.
But for installations with hundreds or thousands of concurrently logged in users that would be a no go.

On our installation (approx. 20.000 users; 800+ concurrent users) we even have restricted polling to 5 minutes or longer.
No complaints so far.

Keeping one connection per logged in user is a resource problem.

which resource would be the limiting factor? memory? sockets? file handles?

on our installation (approx. 200 users) we had complains about the delay :-|

In SOGo each persistant connection would use up one worker, like ActiveSync.
That means it uses up to SxVMemLimit of RAM per user.
Default for SxVMemLimit is 384 MB.
For 200 users this means 75 GB RAM max for SOGo alone.
You realistically need half of it for peak load.
That means you need a machine with more than 37 GB of RAM, as you need some for OS, memcached, database and your reverse proxy too.

Beware, that is for 200 clients.
But each user has normally at least 2 clients (computer/laptop and smartphone/tablet).
With that in mind, only half of your users can be concurrently online.
Or you have to provide even more RAM.