http://www.uboataces.com/periscopes.shtml
It's impressive how many inaccuracies they managed to fit in such a tiny article. I don't think it's appropriate to use it as a reference.
Schnorkeling boats traveled at 5-6 knots with the
attack scope fully extended - the engines make you deaf, you can't be blind as well. They were able to spot airplanes despite the supposed vibrations.
A good reason to not go fast at periscope depth in range of the enemy is that it makes you much more visible.
edit: AP was used on the Type VII boats, on Type IX they could use either AP or OP, so they used the OP