This is a very good (as in useful) review, however, I would say that booking a half-hour appointment and expecting to enjoy 10-15 minutes bathroom time is an error on the OP's part. Making a short booking like this, anywhere, and preparation and post hoc time has to be cut to a minimum, if any is taken at all, because pro rate the masseuse's latency time into the next appointment is going to overrun by dawdling punters. This seems like an elementary consideration.
They almost always cut corners on time at this place. I would make the following points based on what the OP has written and also my own experience there:
1- The OP said the massage time was only around 15 minutes, but I'd be highly surprised if he used anywhere near 15 minutes "bathroom time"
2- After asking you what massage you want and taking the money, they tend to leave the room for 2-3 minutes (first bit of wastage). Upon returning, they mess around with the room (music, adjusting bed, etc). They subsequently undress and ask you to lay down. By this point, at least 6-7 minutes have gone.
3- They typically try and end the massage 50 minutes after entering the premises on hour appointments if you want a shower, and only give a couple of minutes more if you say you don't want one (not set in stone, but those are my usual experiences).
4- The final and perhaps most important point is that your time starts as soon as you enter, and you are expected to leave the premises after 30 or 60 minutes depending on what you have booked. They should allow flexibility; for example actual massage time should be set at a minimum 50 minutes for an hour appointment and if this means the time in the premises is over an hour, so be it. For example, one place i visit (legit, no extras) allocate 1hr 15 mins in their booking system for a 1 hour massage to allow for the admin, changing etc.
Given what they charge at these massage places, I really would expect them not to make bookings which mean customers have to leave the premises bang on 30 or 60 minutes after entering.