ipad air2 runs slowly

My iPad Air 2 now runs very slowly, I think even more so since the last IOS update to 15.8 . It often takes several seconds to wake after pressing the home button, several seconds to respond to the finger ID. And everything takes longer to load or run.  I have already tried resetting with a factory reset and reinstalling a few apps from scratch, but no improvement. It is a 64GB wi-fi model currently with 40GB free memory. My wi-fi is fine - I’ve run Speedtest in the safari browser on this iPad and get 400 Mbps down, 100 Mbps up. 

If I do a forced restart it seems to take a long time to get going; ie something seems to be going on even before it gets to start any apps.

I’ve checked battery usage to see if there is something else running, but the battery usage against apps seems to match my activity.

Should I (somehow) role back the IOS to an earlier version? 

I’ve got an older iPad mini that now runs better than the iPad Air 2.