That's not very scientific. Road bikes are faster on-road than mountain bikes, but does that make them faster off-road too?
If, in riding on the road, the 29er is always 1.5mph (or thereabouts) faster than the 26er, again ridden on the road, on the same route each time, with the same RRalf tyres (as stated in the previous post) then just how scientific does it need to be?! It’s simply a faster bike.
The OP asked a simple enough question, after all, & so all we're doing is taking the time to answer with real-world answers, it's no more complex than that. If, on the back of such anecdotes, people still don't really appreciate that there might be a speed difference, or simply accept the perceived wisdom circa >2008 (originally concluded by some of the bike magazines who are even now notoriously unsure of the whole 29er thang) then no amount of column inches are going to sway the way some people think.
TBH, there’s only one sure-fire way to find out for oneself if, on the trails you enjoy, a 29er would be quicker and that’s to get hold of one; unless you’re happy being told what to think and act, that is; it seems to me that enough riders & virtually every mainstream and independent manufacturer has voted with their design & production & also wallets for to me to conclude that there’s something in this big-wheeler evolution.