When you're doing a Free Kick Set are speedily gliding down the lane like the mermaid you know you are (spish spish spish) or are you really sweating under your cap (THUNK THUNK THUNK) wishing the set was over? Hey I understand thumping air into the water feels powerful, it just isn't fast. When we pound air into the water we decrease the water density around our legs decreasing the propulsion we get from kicking. It's a lot of work for a little pay off. To change this and get your legs more in line, lose some of the bend in your knees until just the soles of your feet are breaking the surface of the water (not seeking perfection on this one.) Once you hit the right kick height for, you, you'll instantly speed up....aaaand shortly after that your quads will start burning.
C'est la vie
On a side note, if you sneak up behind one of your lane mates and thump air into the water with one hand it sounds like something really big just got in the water. Use discretion when using this in a lake, it sounds like a Grizzly Bear just cannon balled into the water behind you. If they're not facing the beach you might be shocked at how far they swim before they even start to slow down. Safety first!