Originally Posted by Road Fan

I would think the cleaning and renewal of all the bearings and the chain is significant, new smoothly meshing sprocket teeth, and a properly set-up drive train are significant. Also if you had a good fitting when you bought it or if your self-fitting is better, that can also contribute. Change to tubeless tires? Hydraulic disk brakes have less drag than calipers?

A new frame can have more or less flex than your older one. Jan Heine who publishes Bicycle Quarterly, holds to the idea (called 'planing") that chainset forces cause frame flex, this frame flex stores energy during the power stroke, and releases it between power strokes to add the stored energy to the propulsion, increasing the pulling tension on the chain. His tests tend to show this is a repeatable and measurable increase in speed. This goes counter to the idea that a stiffer frame has better energy transfer, but I'm not sure scientific analysis or good engineering measurements have come up with conclusive results. If it's a good hypothesis, that probably flex in the bicycle viewed as a system would contribute to added energy delivery.

Also conservation of energy has to be considered. Ultimately physics still holds.

But I have trouble seeing even all of this giving you 20 watts. That's the difference between wins and losses!