Teamviewer QS need too much time to get connected and show ID


we are using Teamvier to support several customers.

One customer has a Problem with Teamviewer QS: It takes up to 1 minute to get connected and showing the client ID.

Is there a way to troubleshoot this? Some kind of logfiles?

I assume some problems with firewall, routing or proxy but until now I can't find the source of the problem...

Kind regards