Execution Quality: Slippage, Latency and Fill Logic
At scale, the distance between your intended price and your filled price is a primary cost, not a rounding error.
At small size, the gap between your intended price and your filled price is noise. At larger size or shorter holding periods, it becomes one of the largest costs you pay — and it is invisible on a backtest that assumes perfect fills.
Execution quality is the difference between a strategy that works in theory and one that survives contact with a live market.
Market execution versus instant execution
With market execution, your order is filled at the next price the liquidity pool offers. There is no requote, but there is no price guarantee either. What you see is an indication; what you get is whatever is available milliseconds later.
With instant execution, the broker shows you a price and fills you at it if the quote is still valid. If the price has moved, you receive a requote instead. You gain price certainty when filled and lose fill certainty overall.
Where slippage comes from
Slippage is the arithmetic of an order book. You want to buy 50 lots; only 20 are offered at the current price; the remaining 30 come from progressively worse levels. Your average fill is worse than the price you saw.
This is structural, not malicious. It worsens when the book is thin, when volatility is high, and when your order is large relative to available liquidity. Understanding it means you stop attributing every bad fill to broker misconduct.
Latency is part of the strategy
Latency is the round-trip time between your platform and the broker's matching engine. For a swing trader holding for days, it is irrelevant. For a scalper holding for ninety seconds, it is a primary determinant of results.
This is also why physical location matters. Traders running short-term automated strategies routinely host on a VPS in the same data centre as the broker's server, purely to cut a few milliseconds off the round trip.
Measure your own slippage
Do not rely on a broker's advertised average. Log the intended price and the actual fill for every trade, then analyse the distribution by session, instrument, and order type.
You will almost certainly find that slippage is acceptable most of the time and terrible in specific, identifiable conditions — usually around news and at session open. That finding is actionable; a single average is not.
Brokers
Compare and find the best brokers for systematic execution.
Key takeaways
- Market execution guarantees the fill; instant execution guarantees the price or requotes
- Slippage is a structural consequence of order book depth, not automatically broker misconduct
- Latency matters enormously for short holding periods and not at all for long ones
- Log intended versus actual fill price and analyse slippage by session
- Market execution fills at the next available price
- Instant execution returns a requote when the price has moved
- Latency to the broker server directly affects scalping viability
- Measure realised slippage by session, not just once
Optimising strategy parameters while ignoring the execution cost the strategy depends on.
Reading is not verification
Take the concept you just read and turn it into explicit rules, then test it. That is the only way to know whether it actually works.
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