trying the same kepub book (regardless of which i try) on my Kobo Mini and one of my original Kobo Aura's 6" both with firmware 3.16.0 vs another of my original Kobo Aura's 6" with firmware 4.24.15676 the page turns on the latter firmware are as the OP describes=slower.
having (only) used firmware 3.16.0 since it was released up until very recently i noticed the slightly slower page turns straight away on firmware 4.24.15676.
can't comment on any of the other 4 firmware's i didn't use them long enough to find out.
best wishes koboy