It's like this: when you or an object shouts, the shouts propagate through space (or whatever you want to call it on a sim) such that every other object within 100m that has a listen has to be checked to see if the listen applies. This creates lag. Normal speech propagates 20m, and so creates less lag. Since it propagates through a 3-d volume of space, and 100 is 5 times 20, it's going to propagate through a volume 125 (= 5^3) times as large as a normal llSay().
A 50m yell, if it existed, would propagate through a volume only 15.625 (= 2.5^3) times that of a normal llSay(). Thus, it would be only one-eighth as laggy, as far as the lag due to volume might be concerned.
I hope that explains it for you; I don't know how else to explain it.
Vixie