We had just the opposite experience at Magic Kingdom last August; waits for the busses to POP at park closing were WELL over 30 minutes! I would have much preferred spending that time hopefully sitting on the monorail, sitting on the tram to the parking lot, and sitting in my rental car, off my feet and in the air conditioning, than standing in the endless queue for the bus back to POP- and then be packed into the busses like sardines, standing the whole way back to the hotel.
SO YES...YES...YES it's faster to have your own car! Easier, cooler, more restful, and more peaceful, too!