Ok, I know that this is another almost impossible question to answer because of all of the variables that come into play, but here goes anyway.
How fast should a person be mowing assuming a tractor/shredder combination that is sized correctly, flat ground with no obstructions and mowing for optimum quality of cut.
I have a TN70D (57 pto hp) with a 7' rhino rotary mower. One 30 acre field I mowed two years ago I was running 6mph and getting a nice cut. Weeds were not thick but were about 3' high. A week ago I mowed 8 acres, again not thick but about 2' high with 1/4-3/8" mesquite running 4.3mph and got a nice cut. In both cases the fields were nice and flat and large enough that there wasn't much backing up or other wasted time. Both were fields that had been cultivated within 5 years or so, so there were no obstructions, rocks or trash to mow around.
Tim
... I usually go 6-7 mph unless its pretty thick.