The jump from air 2 to 9.7 pro was huge, bigger than 9.7 pro to 10.5 proWe have both iPad Air 2 and Pro 9.7 in the household. A lot of folks tend to underrate the CPU improvement and switch to PCI Express SSD. The Air 2 is noticeably laggier compared to the Pro 9.7.
The Pro 9.7 was actually better than the 1st gen Pro 12.9 in some respects (TrueTone, better display, better front and back cameras, 256GB max storage upon release). Only bummer was the 2GB RAM.
Granted, most iPad updates tend to be incremental. The 2020 iPad Pro refresh though felt almost like a non-upgrade compared to 2018 particularly at the 1TB capacity level.
There were 3 major upgrades, the much faster A9X which has basically double the single core performance, the quad speakers and pencil support (which is taken for granted today but was a big leap back then).
Plus several other upgrades like smart connector, much less reflective screen, truetone, cameras and double the entry storage. And this for just $100 more, since it was $599 vs $499.
It's such a shame that Apple didn't go all the way with 4GB RAM and USB 3.0, but I guess it was in order to hit the lower price point, since it was basically the air 3 at the time (there was no other mid-sized iPad in 2016 other than the old air 2 from 2014)