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 sound is 344 meters per second or 770 mph...that of course is in the open air...under water..it's much faster. The speed of light travels 186,000 miles per second...so light is a whole lot more faster than sound. Sound at sea level travels at 1087 feet./second, while light in a vacuum travels 186,292 miles/second. light travels faster. Light travels at 185861 miles per second, at about 760 miles per hour.