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 faster than sound so we see lightning before we hear thunder. The closer you are, the shorter the gap between the lightning and thunder. The speed of sound is around 767 miles per hour (1,230 kilometres per hour). The speed of light is around 669600000 miles per hour (1080000000 kilometres per hour).