Speed up or slow down animation

I have long animation of two waypoints (clamped).
How can I speed up or slow down some parts of this animation?

You can add a keyframe in between the two waypoints and then move it forwards/backwards in time. The animation on either side of the keyframe will stretch/compress as a result. This works even when the animation consists of multiple waypoints.

Only issue is that keyframes apply to all objects. If you want to modify the speed of a single object without affecting others you will need to do it in a separate animation file (Copy&paste layer into new animation, modify speed using keyframes and copy&paste back into original animation).