Hi all,
At first I thought it was a momentary peak of traffic (happens sometimes). After receiving multiple more e-mails and being unable to visit the forum myself I contacted my webhost last night.
Apparently the site received a lot of aggressive traffic from Baidu (which like Google crawls webpages in order to show them in their search results). This resulted in the forum being slow and at some times being inaccessible.
We now blocked all traffic coming from Baidu. The forum immediately got fast again so it seems the problem is fixed.