I was a little hesitant to reply because I don't have the same model the OP asked about. There is still some common ground though. My tractor has three ranges and most of my yard is fairly smooth. Some areas are smooth enough to use high range and run at a pretty good speed. There's one spot on a hill that's too rough to zoom over.
Staying in high range and making an effort to wrap it up quickly, mowing takes about 1-1/4 hours to do 3-1/2 acres. In second range, running at around 2000 RPM or so, it only takes additional 15 minutes. I find myself using second range most often now. It's a lot easier on the equipment - and me. If I save 15 minutes on mowing, I'll likely waste it doing something that isn't important anyway. While I sometimes think it would be nice to replace the 72" RFM with a wider one, it may never happen. To go spend several thousand on a ZTR has no appeal at all.