You initially expressed concern about your speed. 36 seconds for 25m is on the slow side. You say you're doing it in 19-20 strokes. This is just under 2 secs/stroke. That's pretty slow. I'd say if most people who are swimming faster are doing a little better on the SPL and a lot better on the stroke tempo. Try buying a Tempo trainer and using it to push up your stroke tempo while preserving your SPL.
(My sense is that intermediate TI swimmers are doing 25m at 17-18 SPL with stroke every 1.10 to 1.2 secs. What do others think?)