Hello hi2all, welcome to SM

The best/easiest way I've found to slow down/speed up any dynamic simulation, is to bake the sim, then make it a clip in the Trax editor so you can slow it down, or speed it up easily from there by stretching/shortening the layer/clip.
You can even do complex "bullet-time" effects easily using the Trax Editor as well

So:

1. Select your emitter.
2. Go to EDIT > KEYS > Bake Simulation
-Make sure your time is set correct here.
-For example, you can bake the whole timeline, or even just a small part of it, etc.
3. Once your simulation/keys are baked, select them in the Graph Editor.
4. Then go to the Trax Editor (found in the Animation editors flyout)
5. Then, with your keys selected, click on the left-most icon to create a "Clip" out of your whole particle simulation.

Then its all a matter of reading the Trax Editor help to learn what you can do with clips as far as time stretching or shortening goes

Good luck!