Rubbing the hands together is a universal signal meaning that someone is preparing to receive something. Rubbing the hands together is figuratively like washing or cleaning them so that whatever is about to be received is kept clean.
Hand rubbing today occurs just before closing a deal or a sale, when we win a prize or the expectation to win is present. Rubbing the hands slowly often comes across as devious when accompanied with a smug smile. A used car salesman will slowly rub his hands together as he cooks up a plan to pocket a larger commission for himself at your expense. Slow hand rubbing usually means that good is coming to the sender of the message alone, whereas quick rubbing usually indicates mutual benefit. The slow hand rubber is diminishing and concealing his signal by slowing it down or even possibly hiding it, whereas the fast rubber is making his gesture more obvious with his hands in plain view, so both parties can share in the anticipation.