How to limit vertical velocity in OFFBOARD mode?

I am using ROS to control my quad in OFFBOARD mode, and i want to know which params can limit the vertical velocity?

As far as I know MPC_Z_VEL_MAX_UP and DN are the limiting velocities for the velocity PID in z axis. So if you set a small value of those parameters the PID will not command a velocity higher than that so you will get the desired effect of a slower vertical velocity. I do not know if that is the best strategy though, as this will change the way your controller acts, but you can give it a try.