About python : Unable-to-increase-airspeedgroundspeed-of-ArduCopter-in-Mavproxy-using-Dronekit-SITL

I am a fresher in Drone programming. I am currently trying to implement a simple python script in Mavproxy. I am facing problem in setting the airspeed/groundspeed of arducopter in Dronekit SITL. For airspeed/groundspeed set below or equal to 10m/s the simulation is working fine. But whenever the airspeed/groundspeed is set above 10m/s the simulation is not going beyond 10m/s. I am unable to figure out why this is happening. Please help.

Note: My python code is provided below.

from dronekit import connect, VehicleMode, LocationGlobalRelative
import time


def arm_and_takeoff(target_altitude):
    print("Arming Motors")
    while not vehicle.is_armable:
        time.sleep(2)

    vehicle.mode = VehicleMode("GUIDED")
    vehicle.armed = True

    print("Takeoff")
    vehicle.simple_takeoff(target_altitude)

    while True:
        altitude = vehicle.location.global_relative_frame.alt

        if altitude >= target_altitude:
            print("altitude reached")
            break

        time.sleep(2)


def waypoints(lat, lon, alt):
    print("Proceeding to new waypoint")

    waypoint = LocationGlobalRelative(lat, lon, alt)
    vehicle.simple_goto(waypoint, groundspeed=12)
    
    time.sleep(40)

    print("Reached the waypoint")


vehicle = connect("127.0.0.1:14550", wait_ready=True)

arm_and_takeoff(5)

waypoints(-35.36005069, 149.16080384, 5)
waypoints(-35.36526968, 149.16679092, 6)


print("Returning back home")
vehicle.mode = VehicleMode("RTL")
time.sleep(45)

vehicle.close()