I am guessing that the counts of 18 and 19 are not wrong. Based on the whole thread to this point it looks like you were swimming very consistently about 1.3. I get that from 1) the math 2) 1.34 felt slow 3) 1.25 had SPL go up to 26 at the end of the 400. So now as you experiment with different speeds, your body is learning new tricks. I note that you keep reporting trying FASTER speeds. This is intuitive for trying to get faster, but the intuition is not giving you a complete picture. Try slower speeds as well. At faster speeds you are learning how to do what you already do well, but at faster speeds than you used to be able to do it well.
At slower speeds you learn how to do things better.
For instance, as you slow it down you have to use balance more. You also need to maximize thrust since you wonít be thrusting as often, so your catch and pull will likely improve as will your hip drive. Note: When you slow down, the challenge isnít SPL anymore. The challenge is to see if you can keep swimming. I flat out lose it somewhere around 2.5, but my partner loses it at 1.7. You have been so constrained to that 1.3 area that you might lose it as low as 1.4-1.5. My advice is to take it to a speed where you feel awkward and (somewhat) frustrated and work it until it only kind of sucks instead of really really sucks. Then when you go back to your 1.3 you will feel like a genius and you may find that you have a shed another stroke and finished another 1.3 seconds faster.