There are very few instances in life where you'll need to shop as quickly as you possibly can, but don't ever rule it out. I once believed that speed-shopping was only for contestants of America's Next Top Model, when they get ridiculous challenges like "you have ONE minute to style an outfit for use at a black-tie function, but you can only approach the clothing racks while hopping on one foot!"
Then I took on the ridiculous task of lining up for the H&M x Lanvin collaboration. Four hours of sitting outside a shop in the middle of the night... 10 minutes to pull roughly 100 items of clothing from the racks, for six girls, all of whom are different sizes. The strategy was easy: spend four hours memorizing the list, and 10 minutes blindly grabbing whatever is in front of you. Strategy was moot, once the buzzer went off and the clock was a-ticking.
The same applied when online retailer Gizzy & Nacho did a showroom grab-sale: 30 seconds to grab as much clothing as you wanted from their clearance box. It's not a neat box, and 30 seconds is not a lot of time. Inevitably, many came away disappointed -- "What? Time's up? Already?" Instead of spending time picking and choosing, they should have grabbed anything and everything that looked interesting, and even the stuff that didn't. You probably won't want it all, but it's better to put it back rather than leave with nothing.
So the cardinal rule of speed shopping is... forget etiquette. Throw manners to the wind. When you arrive at a warehouse sale, don't consider and then come back -- take, take and take. When you decide once and for all you don't want something, then put it back. Take multiple sizes in case the fit is different from standard sizing. Take multiple colours and patterns. Take stuff you think is cool but not really your style, because someone you're with might want it. If you're with a friend, split up the store geographically so you aren't wasting time walking around.
So really, the best way to prepare for a speed-shopping experience is not to strategize and memorize, but to hit the gym and bulk up those arm muscles. It sounds funny here, but after the H&M experience, my arms hurt for THREE DAYS. This is no laughing matter.