Originally Posted by: SuperhumanJust out of interest I tried a little experiment and set up Cubase with the metronome at 300 bpm and recorded the following exercise pattern (it's not musical but helps to count notes per meter):

Fingering:1234
Root Fret:12
-----------------------------------------------------------1234321
-----------------------1234321------------------------1234-------etc.
----1234321-------1234-------4321----------------1234
1234-------4321234---------------4321--------1234
---------------------------------------4321234

The fastest I could get with accuracy is 12 notes per beat, bearing in mind that the above is a basic exercise and not a complicated run or full scale. It may well be possible to hit 16ths on this particular pattern but I can't see it being possible in regular playing. My point being, it's easy when you are using the same shape up and down but when you start throwing in different patterns on diagonal sweeps of the fretboard (required with four note per string progressions) it becomes impossible. The right hand can only strum so fast and the left hand will find difficulty with changing finger shapes.