Well there are three main factors. First of all your fps, because if you've limited MTA to low fps you can't compete with other players who have high fps. Then there's the ping which depends on your internet connection or your nationality. Keep in mind that servers are physically installed in Europe, so if you're from another continent you probably have an high ping. And finally the NOS usage: aged players knows how to keep NOS for the good spot. For example if you take NOS pickup in a bad zone for acceleration you should wait until the road gets straight or good for NOS usage. I hope that you'll understand which factors influence your car's speed.