It sounds like the drone is detecting one of a number of conditions that restrict the airspeed.
1) Landing - is the altitude correct on the ST16 screen. Log files will be able to record the altitude, have a look through the telemetry files on the ST16. Possibly the altimeter is faulty? May need to calibrate your accelerometer... don't do this in the wind!
2) Stuck in Turtle Mode - Look at the output in the 'Hardware Monitor' in the System Settings button, on the ST16 and make sure the switch is outputting the correct output.
3) RealSence or object collision is active - again check the switch is functioning correctly in the hardware monitor.
4) You are flying in Smart Mode - speed is limited to 13mph (Follow me limits to 43.5mph and Angle Mode 30mph)
5) Use the latest firmware - there was an improvement to the speed of the drone in an earlier firmware update. Got the latest?
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