Rachmaninoff himself didn't use a stiff tempo at all. Please, listen his own interpretation. What a master! In my humble opinion the equilibrium between a "cheese", corny, unjustified rubato and a tasteful, sincere and moving interpretation is very delicate, and its IMHO one of the factors that makes really great the great pianists.
The risk of sounding corny or random may be the reason why many scholars and academicians prefer not to take risks and play (if they can) on an unimaginative and conservative way and, worst of all, they want their pupils learn this way and they try to kill their creativity, instead of teaching how to channel this creativity and reach the equilibrium point. Both extremes are terrible: both a stiff, square interpretation and a so free interpretation that the ear can't follow the rhythm and the coherence of the musical discourse. The ideal, in my opinion, is a middle point, reached by great masters such as (name here your favorite...) Horowitz, Rachmaninoff, Benedetti Michelangeli, Arrau...
I don't mean yours is bad teacher (I don't know him) nor I mean every teacher is bad. But unfortunately there's a bunch of mediocrity on many conservatories classrooms.
This is a piano roll modern (with the limitations of that system) recreation of Rachmaninoff playing of this piece: