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.
Light travels at 299,792,458 meters per second. Light in vacuum travels about 876 thousand times as fast as sound in air. If you meant to ask if light or sound travels faster then this is the answer: Light travels faster, with a speed of 300,000,000 m/s (which is equal to 300,000 km/s, or 186,282 miles per second).