I had a somewhat screwy experience Friday that has brought home to me the downside of slow quotes in the hybrid. At 9:30:19 I submitted an order to sell HRB at 21.26. At the same time a slow quote situation was signaled and a bid posted for 12700 at 21.29. At 9:39:38, 19 seconds later, the slow quote was taken off, I was filled for 400 of my order at 21.29 (out of a total execution of 1600) and the balance of my order was posted as the ask. I did not understand how they executed above my ask 19 seconds later. I called IB, they called the NYSE and were told that the system was swamped with buy orders, implying that perfect order execution could not be guaranteed. I translated that to mean that when a slow quote is posted they can do whatever they want. Meanwhile I never saw an execution against the 12,700 at 21.29 on any exchange other than the 1600 posted on the NYSE. My order would have taken the whole lot. I did see some executions at higher prices on other exchanges. I'd say the hybrid cost me in this case. Presumably I would have gotten those executions under Reg NMS if my quote had been posted promptly. Perhaps I would have gotten the full lot at 21.29, or perhaps not if the size on a slow quote is meaningless. Iâve always thought of slow quotes as a price improvement opportunity for liquidity providers. Now I see the downside.