[Koha] Slow Koha OPAC login process

Hi We're running a new host with Debian GNU/Linux 8 and Koha 16.11.04. Usually the Koha OPAC login process is quick as expected. But sometimes when trying to login it takes 10 to 40 seconds until the login process finally comes to an end. At such occasions the following process can be seen with "top", taking 99% of the CPU resources: phsh-ko+ 59422 56031 11 16:30 ? 00:00:35 /usr/share/koha/opac/cgi-bin/opac/opac-user.pl This happens more often when trying to login from the internal network, and less often when trying to login from the external network (i. e. the internet). Has anyone experienced this problem before and can maybe give us a hint how to tame "opac-user.pl"? Best wishes and Happy Easter: Michael