Modern military arms all seem to have come to some general consensus that the ideal rate of fire for a weapon is roughly between 650-800~ RPM. However at least until WW2 the generally sought rates of fire were much lower, America's first mass issue bolt action simply firing from a 5 round magazine was deemed excessive at first and the Krag was fitted with a device to stop the magazine from working so troops would have to fire in a single shot mode like the old Springfield trapdoor guns. In world war 2 sub-machine guns were often below even 600~ RPM. The MP40 and Sten fired at 550~, the American grease gun fired even lower at about 450~ which replaced the Thompson which itself was modified from it's original prohibition era design to shoot slower; so I could be wrong but that seems like an indication that these are conscious design choices to make the guns shoot slowly, not just the result of them being limited by less advanced mechanical engineering at the time. So what changed that more modern designs like M4s, MP5s, and AKs now shoot much faster than that?