@shurite7 covered much of what I would have to say. Many more factors, including practical needs, go into the length of a dive rather than just the amount of air remaining. The times you cited I think are quite reasonable. PADI has a minimum for duration of an OW training dive, based on either dive time or air consumed. I forget the air standard, but the time is a minimum of 20 minutes.
Also remember that you are an experienced, comfortable diver, not a newbie that is not even certified yet. The instructor probably has a good feel from past experience for not pushing the limits of student divers and wearing them out before accomplishing the agenda for the day. Priority #1 (after safety of course) is to get the course work done. Have fun is a close second, but not the primary objective. Depending on what is going on, that training dive may have also involved another solid chunk of time doing surface skills, and that all starts to weigh on the student diver's energy level and focus on the task at hand.
Regarding DSD, it sounds as though your expectation was a one-on-one instructor to diver ratio. While close supervision and low ratios are a PADI DSD standard, one-to-one is NOT a standard. The divemasters also play a role in head count and supervision. From your description it is not obvious to me that the DSD diver(s) weren't adequately supervised, although it might have been the case.
Also remember that you are an experienced, comfortable diver, not a newbie that is not even certified yet. The instructor probably has a good feel from past experience for not pushing the limits of student divers and wearing them out before accomplishing the agenda for the day. Priority #1 (after safety of course) is to get the course work done. Have fun is a close second, but not the primary objective. Depending on what is going on, that training dive may have also involved another solid chunk of time doing surface skills, and that all starts to weigh on the student diver's energy level and focus on the task at hand.
Regarding DSD, it sounds as though your expectation was a one-on-one instructor to diver ratio. While close supervision and low ratios are a PADI DSD standard, one-to-one is NOT a standard. The divemasters also play a role in head count and supervision. From your description it is not obvious to me that the DSD diver(s) weren't adequately supervised, although it might have been the case.