The official maximum speed of the Hovertrax 2.0 is 8 MPH. This is about average to below average for hoverboards.
Keep in mind that faster is not always better. The faster a board goes the harder it is to control and the harder your fails are when you fall.
I do like how some boards, like the Swagtron T1 and T3 and the Segway miniPRO, have beginner modes that slow the board down to start, but allow for you to unlock the extra fun of more speed as you become more practiced.