Before anything else, just to be clear. I don't know your application, the solution I'm posting is cheap and easy to create but is not safe. During operation there's lots of ways this thing can fail.
First of all, we need to understand the physics in your situation. For an object with 1Kg of mass under effect of pure gravity (vertical acceleration of 9,81m/s²) and the course of 20cm (0,2m).
With our object free falling there's:
- Travel time of t= (0,2[m]*2 / 9,81[m/s²]) = 0,04s
- A force of Fp= 1[Kg] * 9,81[m/s²] = 9,81N
To reduce the falling time we need to control the speed and to achieve this a counter force is needed, and the way I'll show generate this with the friction effect.
You can work with the simplest way, the rope pass inside two pieces
of soft material (like rubber) and you can increase the pressure
with a screw. (this one works exactly like when you use your own tumbs to create pressure and decelerate)
You can work with a wheel apllying friction (the system @Solar Mike sugested if I'm not wrong).
This one I'm not sure if will work, but based on what I understood until now (about electrics), you can use an electric dc motor (operating like a generator) with resistors to control the speed, the more resistance the greater the torque necessary to rotate the shaft.
Below I'll put images of system 2. The 3rd is the same way, but using a electric motor. And the first system I think is kind of self explanatory.
Obs.: These systems aren't failprof and even the best solution. But if you want some crude and cheap solution these options are the best way I know.