15 mph

Look in your settings for “speed limit” or some such. You want to set it as a class 2/3 bike.
15mph is a common limit for compliance in European countries.

Google is your friend.
How about details. Have you owned it a while, and it suddenly started going slower? Or..
You just got it, and it goes no faster than 15 MPH.
If it is the latter, you may have got the European version, OR it is just like Atavar said, you are configured for a low limit.

The last question is, "what wattage is stamped on the hub motor. Your bike is available with a motor as small as 350 Watts.