gun designers can adjust the cyclic rate to whatever they desire (within reason). i don't know how they do it, but the numbers bear this out
It's pretty much determined by and limited by the mass of the bolt and the return spring. A given bolt mass is accelerated to the rear. Its rate of acceleration is determined by the force that compels it. The bolt is then accelerated forward by the spring. Its rate of acceleration is determined by the spring's strength and rate.
Lowering the bolt's mass will cause it to accelerate more rapidly...but the spring will also decelerate more rapidly...so there's not much of a gain there, if any.
Upping the spring's rate will accelerate the bolt forward faster, but it'll also decelerate it faster...so again, little if any gain.
You can design a gas assist to accelerate the bolt at a faster rate reward...like on the German MG42 light machine gun...but you're also limited by the return spring. You can bump the spring's rate to give the bolt and little more forward speed, but you reach a point of diminishing returns when the spring gets too heavy for the operator to manipulate the bolt.
Cyclic rates in blowback submachineguns are generally set and controlled by the mass of the bolts and the return springs. Up the bolt mass with a given spring, and the gun slows down a bit. Lower the mass, and it speeds up a bit.
Another way is to shorten or lengthen the bolt travel. Longer distance=lower cyclic rate.
Back in the lawless 80s...before legislation banned possession of the bolt...an UZI could be converted to select fire by replacing the legal bolt with an open bolt, and removing a small tack-welded plate that allowed the selector to move to the 3rd position.
The idiots who flirted with a 5-year expense-paid vacation at Club Fed discovered that their new toys provided a furious cyclic rate of some 1250-1300 rpms rather than the 600-650 that they were expecting. The legal UZI semi-auto carbine limited bolt travel with a solid steel block, whereas the subgun didn't. They also discovered that the UZI wasn't designed for cyclic rates that high, and most of'em experienced a lot of malfunctions as a result.