If you drop a 10 pound rock and a 100 pound rock from the same height at the same time; they both reach the ground at the same time.

That being said, my fastest speed on a bike (58 miles an hour) was reached going downhill on a fully loaded bike (front and rear panniers and handlebar bag, sleeping bag and pad and tent. The load was approximately 85 pounds. If you're adept at riding with load, the bike is a tremendously stable (downhill) rocket.