If you're a typical swimmer and want to improve your speed, you should take a good look at your kick. Most MOP and BOP triathletes bend their knees too much and let their legs spread too far apart to be decently fast.
Objects that move well through a fluid have a tapered tail. When you're bending your knees and spreading your legs like you're running, you create a low pressure pocket of drag behind you that is actually trying to pull you backwards. You want your tail-end to be more like a barracuda, not like a jellyfish.
A great way to fix this is to keep your legs straight at the hips and knees, together, and kick from the toes instead of the knees. In fact, the ultimate indicator that you are doing it right is that your legs are straight and the joints of your big toes keep barely tapping each other as you kick. If you can do that, your legs are tapered off and now you are streamlined.
When you fix the kick, what you will notice next is that you start wiggling all over the lane, hips going side-to-side with every stroke. What this proves is that you were previously spreading your legs to balance out your learned out-of-balance technique. You were basically swimming spread-eagle to swim straight. When you brought everything in line, your twisty form revealed itself. Not good, right?
When you can to the legs-together-with-big-toe-joint-tapping freestyle without wiggling down the lane like a water snake, then you've officially got it.