Why this decides more than you think
Every time you open and close a trade you pay the spread (the gap between buy and sell) and, with many accounts, a commission. Individually it looks like nothing: a couple of dollars. But multiply it by your frequency and the number gets scary. A scalper doing 100 trades a day can be giving away thousands of dollars a month in costs — a drag their strategy has to beat before earning a single cent.
This is the number-one reason systems that look profitable on paper end up red: the backtest ignores or underestimates costs. An edge of +2 pips per trade evaporates if the spread is 2 pips. So the more you trade, the more cost matters — and that's why brokers love overtraders.
Related: how spread spikes in dead hours, and why we validate with real ticks (which include the real cost) instead of trusting a clean backtest. Learn it in depth in the hidden costs lesson.