Execution speed is the time it takes from submitting an order to it being confirmed as filled, usually measured in milliseconds. Faster execution reduces the window in which the market can move against the requested price, lowering the likelihood and size of slippage.
Execution speed depends on the broker's technology, the trader's connection, and current market conditions — it tends to degrade precisely when it matters most, during high-volatility news events.
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Guide
How Execution Models Work: ECN, STP and Market Maker
Market maker, STP and ECN describe where the other side of a trade comes from — and what each model changes about spreads, requotes and order handling.