As heavy machine guns go, the 550 rpm of the M-2 is considered slow, half that would be glacial. Remember these things are often mounted on vehicles, to be used for suppressive fire, anti-material fire (against light skinned vehicles, like Jihadi car or truck bombs) and even in an anti-aircraft role. In the area suppression role, they are fired in a very short burst, traversed, fired again, and repeat as needed to cover the area of responsiblity.
They were once used as defensive armament for the B-17, B-24, B-29 and even the B-52, in a quad mount in the tail on all but the -H models. They also armed most US fighter aircraft up through the Korean war. The F-86, some models anyway, had 6 of 'em in the nose, to get a high enough rate of fire. (i.e. 3,300 rpm, still only about 1/2 that of the M-61 Vulcan which replaced the M-2 in both the -H model BUFF and many fighters, including today's F-22 and F-35. Although of course the M-2 doesn't throw explosive rounds.
Hmmmmmm, I suppose it could...