Hi Jarrod,
It sounds to me like 2nd ñana / Knowledge of Cause & Effect.
From Mahasi Sayadaw's
Practical Insight Meditation…
"Now, at this advanced stage, mind appears to be the forerunner. The meditator readily notices the intention of bending, stretching, sitting, standing, going, and so on. He also clearly notices the actual bending, stretching, etc. So he realizes the fact that mind knowing a bodily process is quicker than the material process. He experiences directly that a bodily process takes place after a preceding intention… In contemplating regular and spontaneous bodily movements such as the rising and falling of the abdomen,
he notices one after another continuously… One sensation has hardly disappeared than another arises, and he notices them all accordingly. While noticing every object as it arises he is aware that a mental process of knowing depends on an object." (pg. 17-18).
It sounds as though your shift in to "auto-pilot" noting was a shift from 1st ñana (Mind & Body) to 2nd ñana (Cause & Effect). First, one comes to know directly that both physical and mental phenomena are "objects". Then, one notices not only that intention precedes action (and that both arise on their own), but also that noticing occurs automatically as well, and only when there's an object to notice. Things start to speed up, just as you described.
In my own experience, my in and out breaths tend to stutter a bit during this stage ("in, in, in, in, in - pause - out, out, out, out, out.") - almost like skipping a stone across water.
Sound about right? If not, let's get some more info out of you!

~Jackson