FWIW the last time I bought at the ask (earlier this year and with a limit equalling the posted ask) it was filled immediately..

Yesterday, again I had an order in at the ask (with a limit at the ask) and the order was only partially filled... less than 10k shares and the mms quickly raised the ask (and the bid... which had been the ask) so that I was the bid through the morning and part way into the afternoon.

Point is, while they were quick to fill at the ask earlier this year (while advertising a size of 5000), yesterday they would not fill as quickly as in the past at the ask.