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?
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?