Currently SPX monthly AM settled options are treated differently then all the other SPX options. Brokers hold orders that are displayed on the screen, so it is possible that you may not be filled for some time, even when paying the offer. Also if you are a "professional" your orders are routed to a broker, or if your order is over a certain size. The CBOE announced that these monthly options will transition to the same Hybrid system that the SPXW's trade. You will now have multiple quoters for these options and you should see much tighter markets quoted once this transition takes place. Should be good news if you trade the SPX http://www.cboe.com/publish/regcir/rg17-136.pdf
I don't trade AM at the moment but good news for those who do. Unfortunately I think we'll still be subject to the issue where brokers can arbitrarily decide to send orders over a certain arbitrary size "to the floor" where they'll sit, just like they do with the weeklies now? I wish they'd just get rid of the "hybrid" part of it altogether and go pure electronic.
Feb-19 ADV 283,784 Jan-19 ADV 320,117 Dec-18 ADV 466,113 Nov-18 ADV 355,347 Oct-18 ADV 484,042 Sep-18 ADV 302,955 Looks like volumes haven't changed much since 2017, maybe even lower.