I have two Mac computers that I connect to using Screen Sharing and when the Mac Mini's wifi is disabled I get great performance out of it. The problem that occurs is when I enable wifi on the Mac Mini and use screen sharing it appears to bypass the ethernet connection and use wifi instead to connect resulting in a slow connection. Is there any way to force the iMac to connect to my Mac Mini via ethernet instead of wifi when wifi is enabled on the Mac Mini?
In System Prefereces > Network set Ethernet above Wifi. That should do the trick.
Use the System Preferences > Network > Action (cog) menu > Set Service Order to alter the default network.
You can change the Service Order of your network connections. The order tells Mac OS X to prefer one network connection over another.
Drag your Ethernet connection above your Wi-Fi connection: