Several users have reported an issue in which YouTube content downloads extremely slowly, even over otherwise speedy WiFi connections, on the iPhone. In fact, YouTube videos generally download much more quickly over a 3G connection for afflicted users.
A few sample reports from this Apple Discussions thread:"Everything else is fast, but Youtube is so slow I don't use it. If I switch to 3g it's very fast, but wifi forget it." "I'm having the same problem: safari is fast, speedtests show my connection to be fine, but when the YouTube app is running … Read more