> why progress was slowed down for so long.

I’m not sure there necessarily was that much progress before that, though? With some exceptions ancient societies were highly stagnant especially technologically in contrast to high-late medieval Europe.

Also plague, climate change and demographic collapse kind of directly kickstarted the dark ages.

High late medieval times, was when the inquisition lost power.

And in ancient greece there were already concepts of a steam engine. I call that signs of progress not happening for a long time after that.