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 300,000,000 meters per second, but the speed of sound is only about 340 meters per second. If you are watching fireworks that are about a kilometer (1000 meters) away, the light takes only 3 millionths of a second to reach you. The sound takes about 3 seconds. You can tell how many kilometers away fireworks are exploding by starting to count seconds as soon as you see an explosion.