25 Shots per minute??? Holy cow.
This number is missleading.
You propably thought they really did fire 25 rds in one minute or 100 rds in four minutes. Wich is not the case.
Its not even possible. The barrel, recuperator and recoil breakes would heat up to a point were they loose function and get damaged.
The sustained ROF for the Zis-3 is about that of any field gun in the 75 mm/3" class. As a rule of thumb one shot every five seconds. The 25 rds per minute is in artillery mode from a prepared position, firing a single mission with a fixed number of shells. With time to cool for the gun and reorganising after. 25 spend cartidge cases in the firing position are pretty disturbing.
Like all manually loaded guns the maximum ROF that can be achieved for short times is much higher. Its as fast as the loader can ram the shells and the assistant loaders can bring new ones to him. In prepared rapid fire field and AT guns in is caliber can fire one shot every second. Wich would be 60 per minute. But this is theoretical since it can't be kept up for a minute. Maybe for 10 seconds. About 15 max. As a modern example: a manually loaded 120 mm smoothbore tank gun as used by most NATO countrys can achieve a burst ROF of 5 shots in 10 seconds. But is official ROF is 12 per minute.
In addition the effective ROF in AT mode is not so much limited by the loading team but by the ability of the gunner and leader to observe the fall of the shot and adjust fire. This usually takes several seconds. If the target is 500 m away the shell is in flight for ~1 second. Longer ranges increase the TOF. The smoke and dust kicked up by firing obscures the target etc. So in most cases the effective ROF in AT mode will be in the one shot every 4-5 seconds range.
P.S.: I found a nice movie on this
https://www.youtube.com/watch?v=vkCMWe_NrcA
Its in russian but the voiceover is not important. There is lots of original, historic footage. On these the factors limiting the RoF are immediatly apparent.