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 at 300,000 km/second, whereas sound travels at 0.3 km/sec. This is why you can see the flash of lightning sooner than you hear the rumble of thunder that the initial lightning strike produces. As the distance to the lightning strike increases, the pitch of the thunder lowers.