I just wanted to clarify that if you select the keyframes you want to scale, and then press CTRL + M this will bring up a window that will allow you to either multiply (*) or divide (/) the current keyframe spacing. For example, typing 1.25 and then hitting the * button will cause the keyframes to be spaced out 125% of their current spacing and thus slow down the animation, whereas typing 0.75 and pressing the * button will scale the keys to 75% of their current spacing, moving them closer together and thus speeding up the animation.
The plus and minus keys will simply space the keyframes by the specified amount each time you press them either further apart or closer together. This also allows you to select a single keyframe and push it back and forth on the timeline for slight timing adjustments.
You don’t need to do the Transform>Stretch/Shrink part that BjornToulouse suggested as it’s a separate feature.