The wait feels endless. The checkout line hasn't moved in 10 minutes. Why did you pick this line? But you can't risk jumping into another queue that may be slower. Tick, tick, tick. Maybe you don't even want this sweater. Would it be quicker to bail and buy it at home online?

The retail checkout line may be tedious any time of year, but it is worse at the holidays, when stores are packed and people are more likely to wait it out to meet a Dec. 25 deadline than walk away empty handed.

Several...