I would need to hear you play to help you, but generally speaking, Rachmaninoff allows for lots of rubato and each performer's interpretation is somewhat different. It is a romantic music, heart.
As far as getting it faster, my suggestion (without hearing you) is to use a metronome, slowly and build from there.
Begin at say 60 at the quarter and go up 2 by 2 or 5 by 5.
Divide the piece in sections and dont move past a section until the one you are doing is good, sounds good