I have a 12.9" iPad Pro 2nd generation model (2017).
Have been using Affinity photo v1.6 with the appropriate model apple pencil and it has been perfect, smooth and flawless. Just updated to 1.7 this morning and now making any adjustment (especially inpainting or cloning) is excruciating. Slow. Lagging. Very unpleasant.
Is there any way to revert back to v 1.6?
If the fix is to spend $1k + on a current gen iPad pro then that's not much of a solution.
You would normally be correct, however we actually recommend using the beta over the App Store versions for macOS and iPad. As per this comment from Andy S (lead Photo developer) in the beta forum:
'We recommend that you use this beta in preference to the store version if you are affected by any of the issues listed below.'
So please, if you are struggling, I would encourage you to sign up for the beta.