Fastest doesn't always matter in a grocery check-out lane. Look for the cashier who appears to be a Mom. She's the one who won't crush the bread or put the big jar of spaghetti sauce on top of the egg carton at the last minute.
Besides, the better item to optimize when grocery shopping is the actual shopping experience. While you may spend five minutes in a check-out line, you may spend an hour or more shopping. Nicking a minute out of the check-out experience is irrelevant if you just wasted five minutes zig-zagging through the store for multiple items (traveling salesman problem) or if you made a couple calls back home to find out if you were out of ____ or needed ____, a time issue that is magnified if you forgot something and need to return to the store before your normal (optimized) shopping trip.
I wouldn't optimize for the mom. Instead I'd look for anybody who didn't look like a fresh-faced teenager. Since I enjoy living near a grocery store that has reasonable prices, I also tend to recognize the workers. The ones who seem the most senior usually can work the till faster.
You're right about it not really being worth it for optimizing your waiting time. If there is only one checkout and it's some lady with 50 items and she's struggling with a checkbook, other cashiers will usually open up if you make eye contact with a free employee.
You'd save the most time by planning your shopping in advance. Make a list, and categorize it by food type. If you know your store layout, some time spent thinking about the order of you visit can pay off quite well. Plus, it helps reign in impulse purchases.
Besides, the better item to optimize when grocery shopping is the actual shopping experience. While you may spend five minutes in a check-out line, you may spend an hour or more shopping. Nicking a minute out of the check-out experience is irrelevant if you just wasted five minutes zig-zagging through the store for multiple items (traveling salesman problem) or if you made a couple calls back home to find out if you were out of ____ or needed ____, a time issue that is magnified if you forgot something and need to return to the store before your normal (optimized) shopping trip.