I have a Comp (#1) with several layers in it. That comp is embedded in another Main Comp (#2) and I wanted to change the speed it runs at in that comp.
So, I know I need to add the Speed Effect to the Comp (#1) that's going to be embedded, rather than in the actual Main Comp (#2), but I knew I would need to adjust it a few times to get the timing correct and didn't want to have to do that to all the different layers of video in Comp (#1) until I got it right.
The solution was to drop a Grade Layer on top of the other layers in Comp (#1), apply the Speed parameter to that layer, set it to 0.5 to test with and all the layers slowed down together, both in the Comp (#1) itself, as well as in the Main Comp (#2)
The only thing that you need to be aware of is: when playing back the Comp (#1) with speed applied, any Keyframes are in the original positions, not the slowed down, or sped up positions, so they'll appear out of sync with the video when it plays. Turning off the Grade Layer's Speed Effect will allow you to make changes to those properly, then turning it back on again will make it run at the new speed again.
Well, it was news to me, anyway...