I find it very difficult to score good marks in snap because it is speed based i can't complete everything what can i do to improve

Are the mocks for nmat and snap a notch tougher than the actual ones ?

first relax, then analyse from where you can get marks,,, make plan, implent....dont think too much about cut off and all... make sure you are relax.. your performance will be at the best of yours at the d day.