Fees, spread, and slippage are paid on every trade — win or lose. A thin edge can vanish into them.
Every trade pays a toll before it can make a cent: commission, the spread you cross, the slippage on your fill. You pay it whether the trade wins or loses, so total cost scales directly with how much you trade. A genuinely positive gross edge can end up net-negative once the toll is counted — profitable on paper, losing in the account.
The more borderline setups you take, the worse the drag gets, for two reasons at once. Marginal trades have the thinnest edge to begin with, and they still pay the full fixed cost. So each low-conviction entry earns less and costs the same as your best one. Overtrading and cost drag are the same disease seen from two angles — both are cured by taking fewer, better trades.
Compare your cost-per-trade to your average winner. Above roughly 20% of your average win, costs are a real drag, not a rounding error. Fix it from both ends: take fewer, higher-quality setups so each one clears its fixed cost, and cut the cost itself where you can — tighter spreads, better fills, lower commissions.
Reading about R is the easy part. The journal makes you keep the stats — grade the setup before you know how it ends, and hear the one thing to fix next.
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