Natures don't affect how fast a Pokemon levels up. Natures only affect a Pokemon's stats.
What the "best" nature is depends on what Pokemon you're trying to raise. For example, if a Pokemon learns only physical attacks and no special attacks, an Adamant nature would probably be best for that Pokemon since that nature raises physical Attack and lowers Special Attack.
But yeah... natures really don't have any effect on leveling Pokemon up. What affects the rate of leveling up is the experience points you earn in battle, and the amount of experience points you need to grow to each level. Some Pokemon need more than others.
I'm not entirely sure if that's what you were asking about, but I answered as best I could...