That's the exact pace and method I use pretty much so maybe I'm faster than I thought. Sometimes I can speed things up and grab the shirt while it's indexing and slide it onto the belt in one motion and get to 10 shirts/minute but the tack has to be just right and a front print location.
In regards to the counters that are on the press, I've compared several different machines to real world times and none of them have been very accurate. In their defense, none of them were newer style machines so the post 2005 machines could be very accurate for all I know. Our Centurian was WAY off and there were times where it would say we were running at 65dz/hr and we were barely doing 40dz/hr but it would also be off in the other direction just as often.