From experience, if one virtual is slow and none of the other virtuals are slow, then it's not the BIG-IP.
Troubleshooting steps outside of the F5:
Download and install fiddler2. If you need to decrypt SSL traffic, check the help page on setting that up.
Open fiddler2, clear your web cache, and hit the website on the virtual server IP. Highlight the transactions and click the timeline. You can see what is taking so long to load.
Next, modify your local host file to bypass the F5 and go directly to the server.
Clear your webcache, and hit the website again. If it's stil slow, then you know it's the server. Gather the timeline info and foward to your dev/server team. It's on them at this point.
Do this for every server in the pool. It's possible that you have a single webserver that is the culprit.
Good luck.