[saltfish]

As ozzypriest mentioned i think the throttle lag is there on purpose and i also believe that the reason why its there is because its a safety feature. Reason being is that in case you have an emergency situation (panic stop) and accidentally slam on the throttle instead of the brakes, you would not want to accelerate faster into the accident. That delay gives you enough time to notice that you are not stopping as you should be and allow you to readjust yourself to apply the brakes.

This is why if you have a constant feed of fuel you would not experience any throttle lag (not a panic situation) however if you were to close the throttle and then gun it (can be interpreted as a panic situation) you would experience some form of delay.