Back when I used to ride road bikes, I upgraded from a decade old (but very well maintained) aluminium Trek 1000 to a carbon fibre Merida Scultura.

I had the same tyres, pedals, and shoes on my new bike (switched them over). I had a 3-4 km/h improvement on average on the flats instantly. So my new carbon bike was objectively better than my old aluminium bike.

I'm not sure if the improvement was because the bike was brand new, or because it was carbon and had higher end components, but it was definitely there.