Some Sur Ron's shipped power and speed limited so they would be legal for street use in most states.

If your Sur Ron feels really slow this is probably the case.

To put your Sur Ron in "off road" mode for riding on dirt tracks, private property and racetracks, all you need to do is join together the green and black wire that is located under the battery door in front of your seat.

Join the green black wire together any way you can (it is a signal wire and will not shock you) and you are good to ride at full speed.

 Update:

On versions sold mid-summer you want to cut this same wire, not join it.
In other words, do the opposite for X version purchased after mid may-september 2019.

Regardless, if the wire is cut, join it, if it is not cut then cut it. See this page for more info

Here is a video that shows you how to do it: