speed is really the same thing as the impedance problem. It is the impedance of the input capacitance (parasitic and S/H cap) that changes the input impedance , hence the relative limit of the impedance you need to drive it.
This will affect the extremes more than the mid range values since the other side of the s/h is at Vref/2
If you want "typical" values you'll need to specify what you mean by that, then buy one arduino and suitably large sample of replacement atmega chips (preferably from a variety of sources).
I found a notable difference in the two I had available.