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 denser the medium, the faster sound will travel. Sound travels through air at the speed of 1,115 feet per second, through water at 4,757 feet per second and through iron at 16,404 feet per second. Light travels at 186,411 miles per second in vacuum. In water, the speed of light drops to 139,808 miles per second.