Slippage gets blamed for a lot of disappointing trading results, sometimes fairly and sometimes not. Understanding what it actually is makes it much easier to tell the difference.
What slippage actually is
Slippage is the difference between the price you (or your EA) requested for a trade and the price it actually filled at. If an order is sent at 2415.20 and fills at 2415.35, that's 0.15 of slippage — small in this example, but not zero, and it compounds across every single trade a strategy makes.
Why it happens, even with a good broker
Between the moment an order is sent and the moment it reaches the market and executes, price can move — especially during fast, volatile conditions. This isn't inherently a sign of broker misconduct; it's a natural consequence of markets moving in real time and orders taking a small but nonzero amount of time to process. Slippage can work in your favor just as easily as against it, though in practice it's noticed far more when it goes the wrong way.
When slippage gets worse
During high-impact news events, when price can move sharply in the seconds around a release, liquidity can also thin out momentarily, both of which increase typical slippage significantly compared to normal conditions.
During low-liquidity hours, such as very early in the Asian session for many pairs, thinner order books mean less depth for an order to fill against at the exact requested price.
With a broker offering poor execution quality, where slippage is consistently worse than genuine market conditions would explain — this is a real, checkable difference between brokers and part of why execution quality matters when choosing one for automated trading.
Why this matters more for automated trading than manual
A manual trader placing the occasional trade absorbs slippage on each individual order, but an EA executing frequently across many trades experiences the cumulative effect far more directly — a small average slippage per trade, multiplied across hundreds of trades over time, becomes a real, measurable factor in overall performance, not just noise.
This is part of why a spread and execution check at the moment of entry — refusing to trade when conditions are unusually thin or expensive — is a genuinely useful piece of risk management, not just a theoretical nicety. A strategy that trades regardless of current spread and liquidity conditions is exposing itself to worse average slippage than one that specifically checks first.
What a well-built EA does about it
Checks spread conditions before entering, rather than trading blindly regardless of current market conditions — a spread that's temporarily wider than normal is often a signal that liquidity (and therefore execution quality) is currently poor.
Avoids entering directly into scheduled high-impact news, when slippage tends to spike sharply and unpredictably.
Uses realistic expectations in its own risk calculations, rather than assuming perfect fills at exactly the requested price on every single trade.
The takeaway
Slippage isn't a sign that something is broken — it's a real, structural feature of how markets and order execution work. What separates a well-built automated strategy from a poorly built one isn't the absence of slippage (that's not achievable), but whether the strategy accounts for it: checking conditions before entering, avoiding the worst-case windows like major news releases, and being built with realistic rather than idealized assumptions about execution. QMS Trading's EAs include spread and news-timing checks specifically for this reason.