Quote from jimrockford:
No, you often won't get a fast fill on most securities, in that situation.
This is because the NYSE quote will generally lag the true market, for your security, when that true market is running away from your order. So NYSE will often display the best quote, even though you cannot, in reality, execute against that quote. If so, the trade-thru rule will prevent you from hitting the bid or taking the offer at ARCA or elsewhere. You would then have to wait for NYSE to update its quote, so that you can get executions elsewhere. This is a serious defect in U.S. equity markets, and it is a big part of the reason why the SEC has amended the trade-thru rule. The new version of that rule, now called Regulation NMS, will go into effect at some point in 2006, and at that point, auto-ex venues will be allowed to trade-thru slow-moving manual quotes. Then, you will be able to get a fast fill, even when the market is diving or soaring.