The lines at Wal-Mart seem to get longer every month. Picking the right line can be really hard sometimes. I've found that it's not always the shortest line that will get you through the quickest.
You also have to gauge the checker. Sometimes the short lines have checkers that are slow, slow, slow! It's like they are so depressed to be there (and who can blame them) and they are just lazily scanning each item and taking their sweet time putting it into the bag. The only reason their line is short is because of the high abandonment rate.
It's sort of like driving in traffic. When you come up to a light and are trying to choose the best lane, whatever you do, don't get behind a Buick! Even if it's the only car. Anybody driving a Buick is over 80 and guaranteed to be driving 10 under (assuming they even noticed the light turned green). Unless, of course, it's an old Buick. Then it's a high school kid and you'll have no problem.