Odd. I thought the general algorithm was to record a number points where the user traced, interpolate between them (so you don't end up with giant discontinuities), and THEN rubber stamp on the full list. That drawing looks like my attempt a pencil tool when I was using BASIC back in the day.

Of course, if the program can't keep up with that, then the drawing would appear to lag behind the pen.

If I had to guess, I'd say that the pencil uses solid brushes and the "right" algorithm because it performs acceptably in that case, while the brush tool stamps instead because it can't keep up using that algorithm with an antialiased brush.

Interestingly, the pencil tool doesn't have this limitation. Even when turning the size up.