The costs that eat the edge
The Math of an Edge
Spread, fees and slippage compound against you
Every trade pays a toll before it can win: the spread between bid and ask, any commission (the broker's flat fee for handling the order), and slippage when your order fills worse than you expected. On a swing trade held for weeks that toll is a rounding error. On a scalp aiming for a small move, it can be most of the edge — which is why high-frequency styles need far better execution to survive, and why 'I'd be profitable without fees' is a description of a strategy that is not profitable.