I also had slow youtube loading issues on my iPhone 4, iPhone 3GS, and iPad. I found a suggestion to change the DNS setting to OpenDNS (208.67.222.222, 208.67.220.220) and that mostly solved the slow loading issues for all three devices on my Wifi network. I still occasionally run into slow loading now and then, but it seems to be related to specific videos since other youtube videos will play fine.
http://www.opendns.com/
Can't hurt to try. Just change the DNS in the iPhone's Wifi settings.
http://www.opendns.com/
Can't hurt to try. Just change the DNS in the iPhone's Wifi settings.