The server is being used as a testing server. It only has 1GB RAM, and memory leaks will cause it to be filled up after some time of running with multiple players on there. If a process in linux runs out of memory, it resorts to the so-called swapping, where hard disk space is used as memory, which is obviously very slow. A server restart fixes the problem, but not the cause.
I'm working very hard on fixing the memory leaks, which I can only do since now, because identifying the leaks requires real multiplayer usage data, and just now the US server is providing me with that valuable information. I'll fix up those leaks one at a time, and with every fix, the long time performance of the server will get better, until it eventually has no more leak and will run without having to be restarted.