Most MOSFETS start to turn on at around 4.5v, although it's the amount of charge that actually controls the on/off state, not the voltage. At 4.5V the charge builds slowly, so the FET will turn on slowly (propgation delay), but there will be lower current to the gate. If you increase the voltage to 10V, the current to the gate will go up and charge it much faster, lowering the propgation delay, although your now using more power just for activating it.
Basically, the higher the voltage to the gate, the higher the current draw and the faster the field can charge and turn the MOSFET on. The downside is the higher current draw, and the more powerful your MOSFET drivers will have to be.
Most datasheets that I've seen use either 10v or 12v to determine their propagation delays, timing, etc.