Improving how efficiently you swim is critical to learning how to swim faster without wasting energy. Water is 1,000 times denser than air which means there is a lot of resistance to overcome. Therefore, technique wins over force every time, so it is much better to focus on reducing drag and becoming more streamlined, especially in front crawl.
There are 3 ways to swim faster:
Train your aerobic system more - up to 80% of your max heart rate
Train your anaerobic system - lactate threshold heart rate 80% effort or T pace
Improve your economy and efficiency with better technique
The first two are great for most swimmers and triathletes, but there is a limit to the amount you can push your body before you become injured or overtrained (and feel stale), or one-paced.
The third way, improving your technique, and swimming efficiency, has the biggest potential for being a better, faster swimmer.
Measuring your swim efficiency is called swim golf or "SWOLF". You may have seen this metric on your Garmin or Suunto swim / multi-sport watch or similar. It also relates to 'distance per stroke' (how much water you pull yourself over on each stroke).
SWOLF is a great way of measuring where your sweet spot is for pacing and stroke count.
Try performing the following SWOLF Efficiency test swim set:
Count your strokes and time over 50 meters. Add the two together
50m front crawl counting strokes
50m front crawl stroke count minus 2 (from 1st 50m)
50m front crawl stroke count -2 (from 2nd 50m)
50m front crawl stroke count holding-2 (from 3rd 50m) and swim 1-2 seconds faster
Single arm balance test
50m single arm left passive arm in front
50m single arm right passive arm in front
50m front crawl normal
Ideally you should be able to reduce your stroke count on the 2nd and 3rd 50 metres, and then swim slightly faster over the last 50 metres.
This is where your swimming technique comes into play much more. As you try and hold that slightly faster time, and slightly reduced stroke count, you must think about all aspects of your technique - pulling yourself over the water and using it as efficiently and as effectively as possible!
One of the ways we measure this is by regularly using a Swim golf session every 4 weeks as a benchmark for your swim efficiency. The adage "If you're not assessing you're guessing" rings true here.