This thing is brand new, My first Deere, It just seems slower than most, Not that I want a hot rod. I looked at the drive belt seems fine, maybe a tad loose, doesn't look there is anyway to adjust tension , I removed mower deck, didn't make any difference. I guess I have all winter to figure it out. I might try to find somebody else with one and do a comparison.
It is possible that the hydro linkage is out of adjustment and you are not getting full engagement to full speed! Had this happen numerous times with our used machines. There is an adjustment usually on shift linkage, depending on the model transaxle. You may want to have the dealer service it as it is new as you say.
I used a gps unit on hood, sure enough it did 6mph, seems slower, I did notice that if you have full speed and put on cruse control, it slows down, like it's not allowing fulll throttle. I also noticed there is no adjustment on throttle linkage. I guess I can live with it.