For those who come from a running and cycling background, the common thinking is that the higher the cadence the better. If you are able to move your feet or pedals faster than you speed up. Thus, when you throw yourself into the pool the same principle should also apply right? WRONG!
One of the biggest misconceptions that beginner (and even masters) triathletes make in the water is that speed=greater number of strokes.
Instead the exact opposite applies. When you slow down your stroke, but increase the distance and the power of each stroke then you speed up. If you try to sprint a hundred by taking as many strokes as possible you will not only look like Scooby Doo escaping from a ghost, you will not hit your splits guaranteed. The best, most efficient way to hit those fast speed sets is to:
a) lengthen your stroke
b) increase the power of each stroke
Pretty simple on paper, but hard to implement in practice. For this reason, swimmers include DPS (distance per stroke) drills to improve both principles.
DPS is where you exaggerate your stroke to consciously lengthen it. If you do it often enough and correctly, then your regular stroke will also lengthen. DPS though is harder to do than it sounds. Firstly, many triathletes misconstrue these sets and think they should be easy. While you do not want to sacrifice efficiency and form for calorie burn and feeling winded, these sets if done properly will elevate your heart rate naturally since you are putting so much effort into each (efficient) stroke.
Another common mistake is that they turn these into a kicking drill to decrease the number of strokes they take; they take a stroke, kick a bunch and then take another stroke. Although, I am not a fan of the pull buoy, it may be helpful to use one in these sets so that you can take away the temptation to kick a lot. Moreover, a pull buoy will allow you to focus on your front quadrant.
Next time your coach assigns you a DPS drill (or you do one on your own), focus on each stroke and do it right! While you may feel like you are slowing down, chances are you are actually speeding up.