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 much faster than the speed of sound. Sound moves at 343 metres per second (thats about 770 miles per hour!). The speed of light is a little trickier. In a vacuum it is roughly 300 000 000 metres per second (so nearly 900 000 times faster than sound!). However, when light moves through air or glass it gets slowed down a little bit so light moving through glass moves at 200 000 000 metres per second which is still 580 000 times faster than sound.