Carbon fiber?

As I understand it, reducing unsprung weight (which is not very much for a bicycle with no suspension beyond the tires) can have an outsized effect, but actually reducing weight mostly matters for ascending. For non-competitive cycling, other factors seem like they should be much more significant.

A big one, which is banned in most competitive formats, is a fairing. This is much more effective than having a human hunker down and try to be aerodynamic. Even for an upright cargo bike (which is generally extremely heavy), a fairing in front can make a dramatic difference on level ground with no wind.

Carbon fiber isn't better because of strength, but because it's a lot easier to keep somewhat light while making aerodynamic shapes.

The most important is carbon wheels. Lowering the weight of the wheels makes it easier to turn them, especially uphill.

It’s only a slight improvement when accelerating. At steady state, the wheel weight is balanced out (half moves backwards, half moves forwards).

GCN have done some non-scientific experiments on their YouTube channel on this.

The biggest benefit to carbon wheels is you can make them deeper for less weight penalty (vs aluminium) which gives you a significant aero benefit.

It's also very stiff so less power is lost due to the flex of various parts.

Which is great if one is racing but not so much for long distance riding or riding for the joy of it. Most riders are not racing at any level where it makes a difference. A well made steel frame is a joy to ride. They are incredibly comfortable and track exceptionally well, unlike stiff frames.

Recumbents are noticeably more efficient than uprights. In fact any tech or method that's been banned by UCI should make bikes even more efficient.

carbon fiber is a lot less efficient regarding manufacturing though, compared to aluminum. The difference is even bigger with respect to steel or the life time of the materials