Like all electric bikes, the top speed of even the best electric cargo bikes is set at 15.5miles per hour (25kph) for for many countries including the UK, Europe and Australia, when under the influence of the motor.
In the US, federal law states that an electric bike maximum speed when powered solely by the motor and operable pedals is 20 miles per hour (32.1kph)
Other than paying heed to the legal public highway speed limits, there is nothing stopping you from going faster than those maximum assisted speeds – aside from your own physiological limits.