I've tried EVERY "solution" and nothing has worked. Thankfully I found this link which IS the only solution:
http://appletoolbox.com/2010/05/ipad-slow-youtube-fixes/
On your iPhone
Tap Settings
Tap Wi-Fi on the left-hand side
Tap the blue arrow next to your current Wi-Fi network
Tap the DNS field and enter the following: 208.67.222.222, 208.67.220.220
Exit Settings"
I copied and pasted that from the Apple Support forum where I found it. The DNS settings above are from
http://www.opendns.com which is a free DNS service that speeds everything up, especially Youtube. It went from unbearably slow, unreliable and intermittent to purring like a kitten and loads instantly every time now. Try it. It works. All the other stuff (changing encryption, changing/rerouting ports, turning off encryption, etc.) is all a waste of time.