I'm not very good at this yet, and my daughter shot the video with her phone; but it will give you an idea of how much grass I can cut with a Hustler Super Z in a very short time period.
This length of straight line distance I cut is 263' (measured), and I cut four passes in 70 seconds, look at the video time. This is 4' x 6' (72" deck) = 24' x 263' = 6312 sq ft cut in 70 seconds. 6312 ÷ 70 = 90.17 sq ft cut per second...90.17 x 3600 sec per hr = 324,617 sq ft ÷ by 43,560 sq ft per ac = 7.45 ac per hr. cut.
I'm rounding this off to a close number, but for a mower to cut this amount it will be cutting at a speed of 13 mph, at 80% cut efficiency. This is a 6' deck width x 13 mph x 5280 ft per mile = 411,840 sq ft ÷ by 43,560 sq ft per ac x 80% efficiency = 7.56 ac per hr. cut.
This is another interesting number to look at, when you cut 90.17 sq ft per second you divide that by the 6' width of the deck, this means the mower (due to turn time / 80% efficiency) is mowing at an average speed of only 10.23 mph.
I knew people would look at this video and began saying was he cutting grass or just driving with the deck off, which of course would mean your ground speed would increase. I went immediately after the cut video and took pictures to show there was no uncut grass left, while being cut at a very fast speed with blades that have now not been sharpened in 65 hrs.
The first picture is showing the grass height cut and uncut. The second picture is showing there is no uncut grass left, and this is grass that had 1/2" of rainfall last night. The third picture is showing no clippings showing and a smooth cut for the conditions.