Slippage happens because the market never sits still long enough for your order to fill at exactly your displayed price. Between the moment you click and the moment your broker matches you with a counterparty, the order book moves — sometimes by a tick, sometimes by ten. When the move is against you, you've slipped. When it's with you (rare), you got positive slippage.
It hits two flavors of trader hardest: fast scalpers who depend on tiny per-trade edge, and stop-loss runners who get filled at the worst price during a panic spike. Either way, slippage is the difference between theory P&L (what your backtest assumed) and real P&L (what landed in your account).
You set a buy market order on EURUSD at 1.0850. Your broker fills you at 1.0852. You took 2 pips of slippage against you.
What slippage looks like across regimes: - Stable, low-volatility hours — usually 0-1 ticks. Scalpers can plan around it. - News spikes / open / close — slippage explodes. A 1-pip plan can become a 10-pip pain in the same second. - Stop orders — almost always worse than expected, because they fire INTO the move, not against it. - Limit orders — slippage becomes "no fill at all." Either you got the price or you didn't trade.
Slippage is the single biggest reason traders look at a backtest, think "this strategy works," then ship it live and watch the equity curve diverge. The backtest assumed a clean fill at the candle close; reality charged you 2-5 ticks per trade. Multiply that across 200 trades a month and you've eaten 10R of edge that the spreadsheet never knew about.
In TradeOnyx every trade you log carries a real entry and exit price. The platform doesn't model your hypothetical fill — it uses the actual numbers you imported from the broker, so your Profit Factor and Expectancy are computed on the live result, not the dream result. That's the only honest way to measure an edge.
When you're studying a stretched losing month in the Journal tab, scan the trade list for entries where your fill price is materially different from the price in your screenshot. That's the slippage signal: the market is moving faster than your tooling can react. The fix is rarely "trade more" — it's usually trade in calmer hours or move from market orders to limits.