I encountered that the document annotation tool (more specifically the drawing / handwriting tool) is unusable in combination with Apple Pencil on the iPad Pro. At the same time inking is quite decent in the simple drawing environment, i.e. when there is no picture / PDF in the background.
The problem can simply be described as follows: There is a substantial delay before a line starts drawing. Once the drawing starts, inking then seems to be reasonable but is still too slow to catch direction changes (e.g. the small "s" in "test" is just a straight vertical line instead of a curve in the attached screenshot).
This can easily be seen in the attached screenshot.
In black you see my three times intended "This is a test".
In red I tried to verify the delay theory. All red lines are supposed to start at the blue line. You can see the delay in the distance from the blue line depending on the drawing speed. You can also see that inking is okay once the line started drawing, i.e. there are no further gaps afterwards.
It seems that the refresh rate of the annotation tool has not been adjusted with the iPad Pro update last fall.