I agree with thinwater, it's the trim, weight, clean bottom...especially the trim. Once underway, there's not much one can do about the weight and how clean the bottom is.
Sail shaping is an art, and you've got to tinker around with your setup (main and jib halyard tension, leech tension/resulting sail shape, mains'l clew outhaul tension/resulting sail shape, boom vang tension/resulting sail shape) to find the best configuration for given conditions.
All of this depends upon the apparent windspeed and your boat's hull shape / amount of heel with its resulting weather helm and drag created by the rudder countering it to maintain course.
Generically, you can calcuate your particular (displacement hull) boat's ultimate "hull speed":
# kts = 1.34 x (the square root of your boat's LWL), where, LWL is boat length at the waterline in feet.
Enjoy the art !!