I think you are right, I am misremembering, I got my 9.7 pro quite a bit later than my 2015 12.9 (in 2018 I think) and immediately upgraded to 12 and it was reloading on 12, but not badly, I remember I noticed the difference with the 12.9 however, which never reloaded on 12. Reloads became worse on 13 and I left the 9.7 there (where it still is, 13.4).
As for 4GB devices, I don't remember too many reloads on 13 (I remember them on 14 and 15 and 16, they became worse and worse), but maybe I just didn't noticed back then since they were not as evident as they became later.