Is it reasonable to assume that it may be more difficult to get a fill on a spread order (such as vertical spread) at the mid price than to get a fill on a single order (i.e., call or put) at the mid price? I assume that spread orders are often executed by using implied pricing functionality, so that there is no actually a single party that trades the opposite spread at the same time but the exchanges may fill the spread order against several single orders that in aggregate fit to the spread limit price. Therefore, there are more parameters that have to occur simultaneously when trying to trade a spread order at the mid price than when trying to trade a single order at the mid price and thus I am afraid that it may reduce the chances of execution at the desired price. Also, I guess that the more legs a spread have the more difficult it is to get a fill at the mid price (so that it may be difficult to get a fill on an iron condor versus a vertical spread, for example). Also, if I trade single orders and place a trade at the mid price it may be published as the NBBO (as opposed to spread order which does not get to the exchange bid/ask quotes) and the market makers may be interested to fill my order so that the NBBO will continue to be their wide bid/ask quotes. It is true that market makers sometimes may be willing to trade spread orders over single orders since it may be easier for them to hedge their deltas, but if they have to trade such spread at the mid price (and not at their quoted bid/ask) I am not sure they will have an incentive to trade spreads over single orders due to this reason. I would appreciate if you let me know whether my logic is correct. I trade spread orders on long-term options and since I try to trade them at the mid price I find out that the execution is very arbitrary (although I have patience and I try to work my orders over a few days). I want to understand if it is indeed more difficult to trade spreads at the mid price (as opposed to single orders) or I may be doing something incorrect here. Thanks!