There’s a lot of circumstantial evidence that supports 29ers being quicker – most of the top racers use them. Also some physics – if something has less rolling resistance and more grip, it should roll faster and corner better. Larger wheels generally have less rolling resistance over rough ground, and big tyre contact patches grip better – same philosophy is used in cars.
It may all be only a few % better, but at some levels that matters (maybe not to most of us though).
For what it’s worth, I have a 26″ Stumpjumper hardtail – 9kg, and also a 29er hardtail that I have just built up using most of the bits off the Stumpjumper – both bikes are the same weight. The 29er just feels quicker, partly because it is smoother through rough bits. I will probably use the 29er as my bike of choice as it just feels a nicer bike to ride.
It’s taken me a long time to accept the marketing splurge, and I am not one to suffer continual upgrade syndrome (both my FS and HT 26ers are 11 years old and apart from maintenance and adding lightness I have not felt the need to replace), but having just treated myself to a new frame and wheels I have been surprised at the difference.