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 186,000 miles per second. The speed of sound is about 1,108 feet per second, which is 0.2098 miles per second! The speed of light is about 886,559 times faster than the speed of sound!! Light travels at 299,792,458 meters per second. The speed of sound is 340.29 meters per second at sea level. 299,792,118 is the difference of meters per second.