Add Keyframe Easing to Photoshop Timeline Editing

Photoshop's animation tools are quite basic, and there are no built-in "easing" functions for keyframes like there are in more advanced animation software. The easiest and most common workaround is to use Adobe After Effects. It's designed specifically for motion graphics and animation, and it offers robust keyframe easing controls, including an intuitive graph editor to fine-tune your animations. 

You could, and it would be extra work of course, you can manually create a similar effect by adjusting the position of keyframes. Add your initial and final keyframes. For example, set a keyframe for an object's position at the beginning of the timeline and another at the end. To make the animation slow down as it approaches the final position (an "Ease Out"), add several more keyframes closer and closer together just before the last keyframe. The closer the keyframes are, the slower the change between them will be. Conversely, to make an object speed up (an "Ease In"), add several keyframes that are farther apart at the beginning of the animation, and then get progressively closer together.