There are multiple ways of improving your speed in Mad Skills Snocross.
First, it depends a lot on the lines you take. If you are struggling, try studying what your opponents do and copy that. It may be that you're hitting the jumps wrong, and finding a better path through the track will help you nab the win.
Second, you will get a speed boost depending on how you land. In Mad Skills Snocross, the perfect landing is completely flat. You'll see an exclamation, like "Perfect!" or something similar, when you have a good landing and get a speed boost.
Third, you have the option of spending some of your Nitro fuel. If you press the blue button, you will get a speed boost.