How a broker fills your trade - the speed, slippage, and reliability with which orders are routed.
Execution quality encompasses the full journey of an order from the moment the trader clicks 'submit' to the moment a fill confirmation is received. Key components are latency (how quickly the order reaches the broker's system), fill rate (the proportion of orders filled at or near the requested price), slippage distribution (how often fills are better vs worse than requested, and by how much), requote frequency, and rejection rate on limit orders. Together these define whether a broker's advertised pricing is what traders actually trade.
Execution matters most to time-sensitive strategies. Scalpers building a statistical edge over dozens of trades per day need fills within a few milliseconds at tight prices - a broker that takes 100–500 ms or regularly requotes can make a profitable strategy unprofitable before costs are even considered. Automated strategies (Expert Advisors, algorithmic systems) are similarly sensitive, since an EA that enters on a close but gets filled 200 ms later during a move has effectively missed the entry signal.
Brokers vary enormously in execution infrastructure. ECN brokers co-located in the same data centres as major liquidity providers (LD4 in Slough for European pairs, NY4/NY5 in New Jersey for USD pairs) deliver round-trip latencies measured in single-digit milliseconds. Legacy dealing-desk market makers can take 200 ms or more, and that delay is often where requotes originate. Third-party platforms like FX Blue and MyFXBook allow traders to benchmark their own execution data against their broker's claims - something every systematic trader should do during the first weeks on a new broker.