You may have noticed that the website has been really slow in the last month or two. Finally, we figured out what the problem was and I'm happy to say that the speed is back to normal.
The problem started two months ago when a new DSL modem was installed and the website started to work really slow. Jaap and I have done a lot of research to see what happened, including re-wiring lots of cables and switches and network cards... thank you so much! The first suspect was the new modem, a "Fritzbox", but all the settings looked fine and there was nothing in the log files. Traceroute told us that performance was quite decent. Thanks also to Ben from OpenNMS for your help.
In the end, it turned out to be unrelated to cables -- instead, there was a configuration setting on the server ("DNS servers") that needed to be updated. Once that was done, performance was back to normal. A great relief.