I generally do not recommend running a Mac or any other general-purpose server as a gateway; get an external box (commercial product or a spare dual-network-controller x86 box running Smoothwall or M0n0wall or pfSense) and use that to keep the load and the logging off your server, and to simplify your network configuration.
Misconfigured DNS services is the usual trigger for slow Server Admin and Workgroup Manager. (And for similar problems with Server Preferences, though that's not as commonly used around here.)
Mac OS X Server requires valid DNS for many of its services, and things get wonky when that's not available or not working.
Start by issuing the following Terminal.app command:
sudo changeip -checkhostname
You'll get some character chunder and an OK, or some chunder an an indication of a network or DNS problem.
Then to further check your DNS, check IP connectivity with ping (via Terminal.app command line and ping server.ip.address.here and and then ping server.example.com, or via the GUI and Applications > Utilities > Network Utility) to determine if you can connect from the client to the server...
...Then check forward and reverse DNS. Via Terminal.app, dig server.example.com and dig -x server.ip.address.here to determine if those return valid responses, and that the forward and reverse responses match. Via Network Utility, use the lookup command to lookup by name, then by address.
DNS is a prerequisite for most everything to do with Mac OS X Server and one of the first things that must be set up, and if DNS is not configured correctly or if the DNS servers are unreachable or unstable, then Server Admin will usually be one of the tools that shows beachballs.
You can also use Applications > Utilities > Activity Monitor or the shell +top -ocpu+ command to see what's going on; where the activity might be going.
It's possible you've been hacked, but there are also a number of more, um, benign triggers.
