Correct. The effect is roughly halved speed, although running more than 2 tends to have little impact beyond this initial halving.

So if you want to keep playing your game, I recommend setting idlemaster to the "idle all at once" mode so that you can get maximum reward for your time

Uhh... Would that be the "Idle games with more than 2 hours indivdually, then simultaneously" option? Because mine doesn't have an "idle all at once" option.