This compound re time your simulation without altering simulation visually (arcs).
Simply input your (visually) correct emitter speed and gravity speed, then retime with Retime Factor!
Internal algorithm will compensate values for you.
Your simulation will speedup or slowdown, without modify simulation visually!
Just connect outputs to your emitter speed, and gravity factors.
Enjoy.
Video explanation in http://www.vimeo.com/3978018