So the timer starts on the amber?

 

Yep.

The three amber lights come on in a .500 second sequence with the green .500 seconds after the third amber. The timer starts on the third amber. So the goal is to "cut" a perfect .500 second light by anticipating the green.

In a drag race it is still the first person to cross the finish line is the winner. When a race is over in half-a-dozen seconds (or less) then reaction times are critical.

In theory, if the cars are equal, then the race could be won at the light.

OK Flame away
But notice I said "in thoery" and "could be"

 

The above .500 sequence is for a Sportsman Class Tree/Light.

I pretty sure the Pro Light is setup so that after staging is completed, then all three ambers go at once, (and the timer starts) then there is .400 to the green. Then you would want to "cut" a perfect .400

Back to the Sportsman light and our buddies .561 RT

Your "gut feeling" about the "1/2 after the GO! (light turns green) seems slow" was valid.
They just start the timer .500 before the GO!

 

.400 is pro tree. If you see the green your sleeping.