How much impact did China’s $BTC mining crackdown really have on hash rate and block times? 

The short answer - A lot! 

Hash rate fell 45% from an avg of 163.93TH/s to 89.52TH/s in July.

 block times also passed the 20 min mark for the first time - peaking at 23 mins! https://t.co/EalzcsETpX