Quote from FSU:
The SPX is unique in the options world. While other option pits have multiple Market Makers sending quotes, the SPX has only one. This results in very wide quotes as this MM has no incentive to quote tightly. He does not want to get picked off if the market moves quickly. If an order comes in between the markets, all the MM's have an opportunity to get it.
Individual orders can be placed between the markets my brokers/customers. If the order is held by the "book" it can by traded electronically and instantly (as can the market quoted by the single MM) If the order is held by a broker, it will have to be traded with him. This requires getting his attention, etc. This is why some trades a customer is trying to hit may take 2 mins to fill, while others are filled instantly.
The quotes submitted by the designated MM are quite wide, but the "inside market" is usually quite tight. This is the market the options will really trade at. Their width depends on what is going on in the pit at the time, in terms of volatility, number of participants present, etc. The larger traders will call the pit and broker will get them the "real" quote. Smaller traders need to enter an order in the middle of a quote, and slowly move it if they are not filled. Try and estimate the real middle first. For example if out of the money puts are quoted 2 dollars wide, but one of them is quoted 1 dollar wide, it may have a single order tightening that strike so you would place your order accordingly.