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.
According to Physics Web in 'Sound breaks the light barrier,' a professor of physics in Tennessee has designed an experiment which proves that sound can move faster than light. This looks like impossible -- and it is. In fact, the physicist has tweaked some scientific definitions. No sound can go faster than light. But a sound pulse, or more precisely, all the wavelengths associated to a sound, have a group velocity that far exceeds the real physical limits.