In the old days, Ueshiba did not accept anyone without any martial art background as students. If you look at the backgrounds of Mochizuki, Tomiki, Shioda, Shirata, Tohei, Akazawa, you will find that these gentlemen are/were competent martial artists even before joining Ueshiba. I believe the ranks handed out to them were based on their skill set rather than the time they put into training with him. There may be some exceptions which I am not aware of. Basically, we are looking at different set of standards - then & now.
Two quick examples of post war students. Many students, both prewar and postwar, had training in other various martial arts before studying aikido. As Carl Thompson noted, some did not and still were promoted quickly.
6th dan 1970
4 years start to 4th dan.
12 years start to 6th dan.
8 years start to 4th dan.