The chain is the longest it can be and the cable allows the RD to rest on the stop. I'll look into the jockey wheel situation. They aren't sloppy on their bearings and have minimal side to side play but maybe the cog profile is too worn down. I don't have any new ones to compare them to.
Even if the RD is resting on the stop the cable can still be too tight. A too tight cable will cause the RD to lazily come to a stop. When you test the upper pulley rock it...leading edge in, trailing edge out then leading edge out, trailing edge in.