So having cloned the layer, turn off the original layer as you don't want to have the untimed emitter showing underneath the clone layer.

Then apply a retiming behavior to the cloned layer. Navigate to the behavior from the Add Behavior icon in the toolbar. For this example we are going to use Set Speed, but you can use the others too. One great one is loop, you can set the loop duration to keep looping the particle emitter without having to build many multiple offset layers. I have a funny feeling that this might be more useful than slowing down an emitter. Who knows? I'll let you experiment.