Is there any simple way to retime objects or groups that aren't video clips? For example, making a complex multi-object, multi-behavior layer run for half (or twice, or whatever) as long. The only way I know of to do that now is to manually adjust the in/out points of every element and scale the keys for any keyframed animation... is there no better way to do this?
If you make a Clone Layer of the layer or group you want to retime, you can then apply a "Set Speed" Retime Behavior to the clone and adjust to whatever you want. You'll of course want to hide the original layer or group.
It works reasonably well.
I'm not entirely sure why you can't apply the same behavior to the original layer or group but you can with a clone of it.
If you want to have fun you can also apply the other retiming behaviors in the same way, for example if you want to ping pong a complex animation or strobe it or whatever. And it's also a good way to stop or pause an animation by using the Hold behavior.
Or you can use the Reverse behavior to reverse the timing of your animation - very powerful.
And you can use pretty much all of these retiming behaviors in combination with the same method.