It is pretty safe to say that After Effects is an expression driven application. If you need to create or set up anything complex in After Effects, chances are expressions are how to get it done.

a cool expression to determine a speed of a layer

This isn’t necessarily a bad thing (aside from expressions evaluating on every frame which can make things slower) as Ae’s Expression engine is very powerful.

Here, Mikey Borup shows how you can get the speed of a layer using the Speed Expression in Ae.

Why would you want to get the speed of a layer at all? Being able to find the value of the speed a layer is animating can be useful to pipe into an effect.

In Mikey’s example, he takes the speed value of a layer and uses it to fake a motion blur effect.