Setting object time-scale worsens performance?

I noticed while testing a new mechanic within my game, is that when I've got many objects time-scale set to quite a low scale and they're all moving slowly, that performance actually goes down. Why is this? The objects are moving slower, shouldn't that make the performance better?

btw, this is on mobile.

I'm probably gonna get a good reason. Can't wait!

Is this because you've noticed stuttering, or have you actually measured a framerate difference? I couldn't reproduce this happening, with setting the global timescale and the object timescale yielding no difference.