Think of a kid going down a slide. If the slide is at a certain angle, the kid will go down at a certain speed. If you want the kid to slow down, they can either put their hands out and try to slow down (not very efficient), or you could reduce the angle of the slide.
The same with aircraft. There can be a bit of extra drag available through various means, but it is not efficient. The best way of controlling the speed (assuming the aircraft is already at idle thrust in descent) is by controlling the angle of descent, which is easy to do. The usual phrase is that you can't get an aircraft to go down and slow down at the same time...