"inside knowledge" of the game guide.....
As well the delay time is also tied to the experience and morale ( and health. ) of your gunners. A low exp and mor unit might have, for example a 5.2 delay but a unit with high exp and mor might be 3.5 AS WELL AS the observer unit that is calling the fire. An experienced FOO and gunners will work better and faster and more accurately than ones with less experience or who may have taken damage.