The decline bench press gives you have a limited range of motion. You can use more weight because of both mechanical advantage and less total work being performed. By only bringing the bar down to your lower chest, you severely limit the distance you move the bar.
The time difference that you sense is due to the way sound and light travel. Light travels extremely fast (300,000,000 m/s). In fact, it is faster than anything else. Sound travels at a measly 343 m/s through air. Therefore we can see light in an instant, but it takes a while to hear thunder. Sound has another disadvantage because it tends to bounce off molecules in the air. This makes the sound travel in all different directions.
The speed of light is roughly 882 thousand times the speed of sound. -- Sound takes about 4.7 seconds to cover 1 mile. -- Light takes about 0.0000054 second to cover 1 mile. -- In the time it takes sound to travel one mile, light travels 881,741 miles. During a thunderstorm, you see the lightning before you hear the thunder, even though they're caused by the same event. Another clue to an easy answer: Nothing is faster than the speed of light.