Abstract. 1. Introduction 2. Market Structure: Overview of Technical Details 3. Data 3.1. Sources 3.2. Measuring O-A Latencies and FIFO Ratios 3.3. Determining Speed Races and Excess Messaging by Way of Rapid Order Cancellations 3.4. Summary Statistics 4. Empirical Results 4.1. Order to Accept (O-A) Latency and FIFO Ratio over Time 4.2. Implications for Market Quality: Excess Messaging and Rapid Order Cancellations 4.3. Implications for Market Quality: Unabsorbed Order Imbalance at the Close 5. Discussion and Conclusion References