Which part is slow? Is it slow to resolve to the host? Is it slow to draw content? Once a page loads, is it slow to modify a page? How much data is on the machine and is Spotlight done indexing? Are you connecting with FQDN or with .local address? If FQDN is your DNS set up properly?
Try this...
1: Get a client right next to the server.
2: On the server, view the web/wiki logs using either Server.app, Console, or tail in Terminal
3: On the client, quit Safari, launch Safari, and then try to access the web/wiki
4: How quickly from hitting the Return key in Safari does the log file respond?
5: Are the access speeds the same if you try connecting to the server using http://localhost on the server itself?