It's pretty much an impossible challenge- because you can't change wheel size without changing a stack of other factors, so there's no way to isolate it. (so for example you get boneheaded MBR commentary on how amazing 650b is because it works so well in the completely different new Orange Five compared to the old 26 inch one). But even if you could isolate it, that'd still not be a fair comparison because you'd end up with a basic spec that favoured one wheel size or another- so frexample convertible bikes, which were designed for 26 inch but now have adaptors to take 650b.
So what you're left with is a lot of new model vs old model comparisons, and no surprise, new models are often better, and almost all the new models are 29 or 650b with most being 650b right now. So the best you can hope for is comparisons between either broadly similiar bikes, or the best you can do in each wheel size.
But even then, since there's no such thing as a best bike, everyone'll probably end up going fastest on the bike that suits them best not the bike that is best.
Still there's been some good attempts, the mad scientist Nico Vouilloz tested 26 vs 650b and decided to his satisfaction that 650b is fractionally faster. This is a guy who spent hours machining down and testing different maxles to see which has the preferred amount of flex, after discovering Lapierre had made their bikes too stiff, so marketing aside I reckon if he says it's better for him, it is. But that's only 2 sizes.