I set up a local DNS server (because the router of our ISP doesn't support local DNS) and configured it to resolve to static IP addresses and forward other requests to the ISPs router. So the DNS works fine but now other clients which are trying to connect to other srvices on the server need a lot of time to connect. For example sshd and apt-cacher are very slow. While ssh works fine and fast on other servers. So it seems that the server is too slow to run that many services.
So the final question resulting from that is: How can i find out what slows down things that much?
The server is running on Debian jessie. The DNS server is bind9.
After some research in the syslog i found that bind9 is throwing a bunch of errors like in the following form:
Dec 28 18:36:18 cache named: DNS format error from 192.168.2.1#53 resolving www.arcor-online.net/A for client 192.168.2.3#48110: reply has no answer Dec 28 18:36:18 cache named: error (FORMERR) resolving 'www.arcor-online.net/A/IN': 192.168.2.1#53