Pencil Lag (iPad 2018 vs Pros, Notes vs OneNote, iOS 11 vs 12)
Based on viewing the drawing footage in slow motion, it appears the new Apple Notes app in iOS 12 is using Apple's Predicted Touches technology, which makes an educated guess as to where future touches on the display will appear so the developer can temporarily pre-draw the path there. This explains why the latency now appears faster than OneNote, which doesn't appear to use this technology. As far as I'm aware, previous versions of Apple Notes did not use this technology.

If you want to reduce latency for Apple Pencil input, the 2nd-generation iPad Pros are the way to go because they have the 120hz ProMotion display, versus 60hz for the 2018 iPad, though they are now well over a year old -- so maybe best to wait for an update. Alternatively, you could use PDF Viewer by PSPDFKit, which has the lowest latency of any app I've used because it uses more predicted touches than Apple Notes.