You're oversimplifying it. At some places (eg Subway), it was about 1 second timing for credit cards. At most stores, you would have to do things like say whether it was credit or debit, tell it yes/no to cash back, and tell it yes/no to whether amount is OK. All of them seemingly do it a different way. So, if you're used to it, the process might take several seconds. If you're not, significantly longer. Under Chip and PIN, you have that plus a several second wait in most places I go.

Cashiers with decent performance will take a solid bill like a twenty and give you back your change in a few seconds. If you want to give specific amounts of change, that adds time but so does when you split credit cards to put specific amounts on them. If anything, the cards can take longer since the transactions are separate. Wise users of cash also already have most of their money ready for payment by the time the order is totaled since it saves them time.

So, for most orders I see, the cash and credit take about the same time if customer is moving decent speed. The only difference is if the customer is moving slow for whatever reason (eg indecision or fumbling for stuff). That can happen with either but happens more for cash since credit users look for one item (the card).