In Fallout 4, every weapon has a "Fire Rate" statistic. How does this statistic translate to rounds fired per second?
The Fallout wikia provides a helpful explanation of Rate of Fire on this Weapons page.
The article explains:
"The in-game given number gives the maximum cyclic number of shots/attacks per 10 seconds, without considering reloading.
This is internally derived from the weapons attack delay rating divided by its speed rating. However, if a weapon is converted to automatic or bolt-action fire, the time between shots becomes derived purely from its speed and/or firing animation duration."
If that second part is unclear let me use some more definitions. From a Creation Kit forum post:
"An Attack Delay limits rate of fire by requiring a specified amount of time to pass before the next shot can be fired. The Automatic and Bolt Action Flags disable Attack Delay.
Speed affects the maximum rate of fire and controls the speed of the firing animation (normally). Higher Speed values than 1 divide the Attack Delay and lower Speed values than 1 multiply it."
In simpler terms, Attack Delay is a weapon's charge time, and Speed would be rate of fire if there was no Attack Delay (and no reloading).
To provide some examples:
2 Speed and a 1 second Attack Delay (1/2) makes a semi-automatic weapon fire a shot every 0.5 seconds.
0.5 Speed and a 1 second Attack Delay (1/0.5) makes a semi-automatic weapon fire a shot every 2 seconds.
Attack Delay/Speed returns Seconds Per Attack.
10 (seconds)/Seconds Per Attack returns your Rate of Fire stat and is what's shown on your weapon's description.