As some have mentioned in this thread use D key or C key to dive. The dive angles is increased and the time is decreased. Only hit the P key for periscope depth if you are already under. All boats dived first then made way for periscope depth or whatever depth they wanted to obtain.
PS. Besides that the IX is a big old girl and a nice target for the aircraft but she carrys a lot of torpedoes and fuel. If you play her with the abilities she has your patrols can be very successful.