I purchased Designer yesterday after reading many good reviews about it and examining its abilities and UI. I'm still pretty new to the app, and although it looks very promising, something struck me from the very beginning. As explained in this thread, the vector drawing is initially pixelated (rasterized) then converted to vector. Even if for a split second, it has a huge negative impact on the drawing experience in my view. I already have Concepts and Vectornator apps and none of them appear as such. Affinity clearly has not been able to utilize iPad's (at least iPad Pro's) full hardware resources. I think this is fixable in future updates (hopefully soon) before people like me start mentioning it in their App Store reviews.
I have iPad Pro 10.5 with iOS 11.4.1 and Apple Pencil.