I was a swimmer and raced as a teenager, nothing special, local club stuff mostly. In the days before goggles and touchless flip turns. I no longer train in water, just bands, weights and a few pool swims before the season begins, mainly because getting to the pool is a pain in the arse. I swim just under 2:00 per 100m Open water for 1500m, on calm water it’s closer to 1:50. I could go 1:40’s for 1500 when I was getting ready for IM distance four years ago and swimming once a week in a 50m pool… (take 10 seconds off for yards). I’m 75.
There are two things that allow that, technique, once learned never goes away. And while my stroke form is literally ancient, it was fine in it’s day.
If you watch more than a few swim videos of both men and women you will see that the technique that works best is the one each individual does best. The best example I would give is to watch Katy Ledecky race against Summer Macintosh. Very different arm and entry styles (no real underwater shots), but they are different because Ledecky "gallops and has a two beat kick, Macintosh is smooth.
So find your rhythm and fastest stroke style and make it work for you. I personally believe that if you don’t swim fast, you can’t get fast. The rest is aerobics.
The second is simply… muscle. No one appears to talk about that in their recommendations, but reality is that a “Swimmer” has a vastly different set of muscles than the none swimmer, gained from all those years of swimming. Build some swim muscles, out of the water, it’s quicker. Nearly every top swimmer (yeah some don’t) spends time in the weight room (as do I). If you can’t “lat slap” you ain’t got them.
Cheers