You have highlighted the problem I have with the MMS. It definitely slows down your gameplay, especially if you are ADSing while advancing and looking for bad guys around the next corner.
It is a different type of attachment, and one that has some nice benefits when used correctly. When you spot that guys outline crouched behind a wall you were going to go by, it brings a smile to your face, because you know you are going to ambush the ambusher. Like some have mentioned, smoke is a friend of the MMS.
I made a class up to try out how much of a knob I could be with the MMS. PDW with MMS/Silencer, perks of choice, smoke + bouncing betty. While defending a Hard Point, you know that the enemy is going to try and swarm you eventually. Just before they would rush in, I would drop a betty and lay down the smoke before going to some sneaky vantage point and commence to ADS. I was surprised at how effective it was in some lobbies (some maps/lobbies it was pointless). Kind of nooby knobby, but it was a fun experiment.