I applied some standard optimizations as background apps refresh off, Siri indexing mostly off, Bluetooth off, location services off, airdrop off, AirPlay and handoff off, notifications off for most apps. Things I don’t need anyway.
With that I find it totally useable. 9th is much snappier but doesn’t fundamentally change my experience.
these are things I have done in low end iPads and has never changed responsiveness dramatically.
As for totally usable, yes, if you are patient enough... My mini 4, which is not much different from my (now sold) air 2, is usable if you are (very) patient, but the issue is that we (at least some of us) get used to faster devices and become less tolerant to waiting for things to load (those 3, 4, 5 etc seconds to load and opens things quickly become annoying)
At this point even the 9.7 pro, which back in the day felt enjoyable and so much better than the air 2, is feeling sluggish and really my new good enough baseline has become the 10.5. So A10X, A12, A12X and of course M1 all feel fast enough that it does not change my experience (other than reloads due to RAM), anything below that feels laggy and annoying for general use and browsing... unless I am using a specific app.