There is no “speed” to change. There is the amount of change between the keyframes or the amount of time between the keyframes. You were properly told that to “slow down” the change, make the keyframes farther apart or the amount of change smaller.
This may require adding more keyframes to achieve what you want to do. For example, I’m wanting to go from X to Z over thirty seconds. However, I’m wanting the last little bit to go slower. That’s where you’d add a “Y” keyframe. Once easy way to do this would be to place the “Y” keyframe close to the “Z” keyframe. Then move the “Y” keyframe left so that it’s farther away from Z.
If you do what I just mentioned, you will then see that the speed between X and Y is now faster than the speed from Y to Z. Then you can change the type of keyframe Y is to smooth the transition from the fast speed to the slower speed.
So essentially, start like this:
FFFFFFFFFFFFFFFFFFFF
X Z
You have two keyframes with him falling fairly quickly. Now add keyframe Y
FFFFFFFFFFFFFFFFFFFF
X Y Z
Now move the “Y” keyframe left
FFFFFFFFFFFFFSSSSSSS
X Y Z
“F” = Fast, “S” = Slow
Edward Troxel