The original Apple Pencil and first generation of iPad Pro boasted of a 20ms lag. That was considered impressive, but perceptible if you look at your writing carefully.

I found it fine for note-taking, but lots of people would still notice the latency, especially those using creative tools where there is a strong feedback loop between what you see and what you draw.

They are now claiming 9ms lag. I suspect this is imperceptible for the use case of note-taking and marking up PDF documents (e.g. highlighting, making notes in the margin).

But then again, 20ms is going to be more than fine for that use case as well.