Controlling ground speed in PX4-SITL

As we have a custom uav model, which is based on standard_vtol, I was trying to reduce the speed of the UAV through QGroundControl by changing MPC_XY_CRUISE, it isn’t working as the velocity still is quite higher than the given value. I changed the MPC_XY_VAL_MAX as well but it didn’t work. How am I supposed to change the velocity of the UAV?