I've been seeing 500-series error messages off and on, so I'd say this is probably the server hosting the forums having issues. Since it's still responding (slowly), I'd guess that it's maxing out one or more resource such as RAM. No idea what exactly is causing the problem though. Could be anything from too much traffic to a memory leak in a running program. It's also possible that the server is hosting multiple sites and traffic associated with one of those sites may be overloading the host.

TL;DR: The server seems to be clogged up, which makes it take forever to respond