Quote from JohnGreen:
Actually, getting fast fills may be a matter of matching orders unless they are at the natural bid or ask. If you deal with a broker like thinkorswim or IB, you can be matched internally by the broker. In that case, round numbers are a bit more likely to be filled. Smaller numbers are also slightly easier to fill than larger numbers depending on the liquidity of the options.
When I trade SPX, I often get better prices and faster fills on 5 lot condors than on 10 lot ones. Sometimes, you will not get a fill on a 10 lot at all, than reduce the size to five and it fills right away.
A person who trades options regularly in a particular issue will get a feel for what a decent price is for a certain kind of spread. For instance, I do a lot of condors on the SPX. Sometimes I will see thinkorswim display a certain price as a midpoint price and I will say to myself that is not a good deal. I'll then offer less than the mid, and still it will fill anyway because it is the right price or someone else will offer up for it seeing the same divergence.