Originally Posted by
NitroWD
if high idle takes a longer time to drop, it's not as rich as one which the high idle takes a lesser time to drop.
A dropping idle can also be caused by just cooling down the engine which is atomaticly done at idle when getting the right/rich mixture. The cilinder sceezes again giving the piston more friction and so slowing it down.
Depending on the status of the P/S set the effect can be more or less noticeable.