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 formula is to divide the number of seconds by the speed of sound in feet per second. The delay between when you see lightning and when you hear thunder occurs because sound travels much, much slower than light. Sound travels through air at about 1100 to 1200 feet per second (our default is 1150). That is a little more than one mile per five seconds.