That's a myth itself that scalpers have to be fast. You are given stop loss and take profit options available so to quickly set it up even before opening a trade. But calculating a stop needs time or understanding of the trend or pattern + Risk:Reward ratio, and if someone's scalping, it's better not to put a very tight stoploss because it might get hit quickly so even if it goes back to the trader's profit area, he regrets later. Scalps are for mostly 5 - 10 mins, and that's enough time for a trader to either set or trail a SL once he sees his position in profits.