Those savings are easily justifiable over 21 miles on different bikes. 5 lbs saves 1.5 minutes alone on a course with some elevation. The new bike is likely set up as more aero with lower bars, higher seat, etc., which will save another 1.5 to 2 minutes. Finally as you noted more gears and easier shifting likely kept you in your optimal cadence zone. Not only does that make you faster in itself, you avoid the big slowdowns from having to mash and being fresher in those last few miles.