The "Quick Sale" feature found on page 3 of Store Information Defaults makes two changes to speed up the sales screen when it is turned ON.
1) The sales screen assumes the next sale number is correct and so it immediately skips to asking the customer name (for those times when the sale number needs to be changed, it is easy to press the up arrow a couple of times to chnage the sale number). [With Quick-Sale off, the sales screen starts by asking you to confirm or type in the sale number]
2) After you type a Sku number, the screen immediately skips to the next line item of the sale. I.e., it assumes you will be selling a qty of 1 at the standard price and with the standard description. For those times where you want to change the qty, price or description, it is easy to press the up arrow to edit the previous line. [With Quick-Sale off, the sales screen will ask you to confirm the price, description and quantity of each item after you type the Sku number.]
Effectively "Quick Sale" speeds up the sales screen if a majority of the time you accept the next sale number and accept the standard information with most of the Sku numbers.
On the other hand if you usually change the sale number or usually change the price, qty or description of the line items, turning on quick sale might actually slow you down a bit since you would usually have to press the up arrow to change the information.