Don't equate big air with speed. In slalom, if you make a good turn and a big pull, you are moving like a torpedo into the wake. In wakeboarding, though, you don't need to go that fast.
When wakeboarding, water skiers tend to be in too much of a hurry to generate speed. You see you're only 15 feet from the wake and you're going 20 mph slower than you are used to. You think to yourself, “There's no way I'm going to get any air going this slow,” so you dig in hard. Problem is, you dig in so hard that you can't hold it through the wake, and by the time you are at the critical zone, you have no edge or rope tension. You are going fast, but you have no power. Consequently, you barely get any air.
Almost everything you do on a wakeboard should be based on the progressive cut, having good edge at the wake and getting a little pop off the wake. The progressive cut, or “edge,” is just like in slalom where you want to build your pull all the way through the wake. To get big air you need to have the board on edge when you reach the wake and have maximum tension on the rope as you pull through the wake. You must, I repeat, must build your cut from a drift in at the beginning.
So for the proper cut, pull out moderately wide, maybe half as far as you would if you were starting the slalom course, and then let the board drift in toward the wake. Once you start coming in, gradually increase the pressure on the rail and rope until you get through the wake. This is essentially the same feeling as you should have on your ski. The difference is, as you hit the wake, don't slice through it as you do on a ski, but use the shape of the wake to pop you in the air. The instant before you are through the wake, stand up semi-tall and rigid. This will give you that big pop off the wake without causing you to lose speed.