Does it go faster than that when the wheel is off the ground?
If the speed is identical under load (riding) and offground, then there is a speed limiter in the controller. To fix that, you'd have to see if the controller is programmable, and see what settings it has for that (either via it's display, if any, or via a serial/usb cable to a computer to software specific to that controller, etc). It's unlikely to just be a speed limiter wire, but you can check the controller manual for that, if one is avaialble.
If the speed is higher offground, then it is probably just the voltage of the system not being high enough to drive it any faster for that particular winding of that particular motor vs the wheel size it's in, *or* not enough current from the controller to enable pushing it any faster against the wind resistance, load, and any slope.
To fix the voltage issue, either of those, you'd need to either replace the motor with a different winding, use a larger diameter wheel/tire, or increase the battery voltage.
To fix the current issue, you'd need a bigger (higher current) controller, or if it is programmable, you'd need to reprogram it by whatever method it supports, for higher current limits if it supports that.
If you have to increase the current, then if it's more than the battery supports, you may ahve to replace that, too, or add another in parallel, so voltage sag doesn't get too bad and/or excess current draw doesn't damage the battery.
