HDS 2135 ground speed?

[yardiron]

I picked up a 2135 the other day at an auction, it had low hours and its a bit banged up but it runs and drives.
The first thing I noticed the minute I fired it up and drive it on my trailer is that it seemed fast for a small tractor. I also noticed right off the bat that it doesn't stop when I let off the forward pedal. It coasts to a stop and not the usually abrupt stop when I lift off the pedal.
The ride from the auction house down to the truck was through a maze or cattle gates and block walls, I noticed right away that it seemed to go faster than was 'comfortable'.
The next day I tried clocking it against my truck and got 16 mph, (I got 16.8 mph on my hand held GPS).
I've got other machines with a hydrostatic drive, but none are this fast. I even read in a post somewhere online where someone was complaining about these not having enough ground speed.
This thing is fast enough that I'd venture to guess it would flip if you turned too fast, and the steering on these things isn't really precise enough to go much faster than walking speed.
My question is, what on earth would make one of these go faster than stock?
The tires are stock 20x9-8, the engine is a bone stock 12.5hp Kohler Command, and its shaft drive so there's no pulleys to have been changed.
Its not go-cart fast but its fast enough that if you hit something, it would most definitely hurt.
The fact that it don't brake when you let off the forward pedal is also a concern in that you have to consciously lift your foot and step on the brake to turn or stop. I did find that if I tapped the 'reverse' pedal, it stops coasting and stops hard. I also found that doing that if you do that at full speed, you better be ready for the result. I'm not talking slamming down on the reverse pedal, I just tap it lightly and it stops coasting and screeches to the stop.

At full throttle, if I mash down on the forward pedal, it'll spin both rear tires for a few feet on grass.
I've had these around here before, maybe not this exact model but never one that moves like this one does.

The deck is trashed, bent beyond repair, someone hit something hard and turned it into a boat anchor but I wanted it to move trailers around the yard and for hauling firewood up to the house, so it'll do just fine for that. I wish it were a heavier duty machine but for what I paid I cant complain.
I suppose if I stumbled on a good used deck, it would also cut grass again The hour meter is only showing 119 hrs.