This is real basic After Effects keyframe animation. With the time remapping keyframes at their original distance the speed will be constant and at the same rate as the original footage. If you select the last keyframe and drag it to the right the clip will slow down. Drag it to the left and the clip will speed up. If you slow down the clip you will have to extend the out point of the layer. My suggestion would be to pick points in the action where you want the speed to change and insert keyframes there. Then if you grab and move a pair of keyframes to the right the speed will remain constant between the pair but speed up to the left of the first keyframe and slow down to the right of the first selected keyframe.
If you open the graph editor you can get a visual representation of what you are doing. You can even ramp or ease the speed changes. It just takes a little practice.
I would suggest that you type Time Remapping in the Search Help field at the top right corner of AE and study up on some of the community resources that you will find there.
I want to put the smile from the first screenshot (the smile is the bottom drawing, the face is there for perspective) onto the face in the second screenshot. I only made one drawing in Photoshop but I know that I need the complete set for mouth rigging as in the third screenshot. This the same basic workflow and basic character animation. Thanks.