How many anecdotes before it's evidence?
I took my 29er duallie to Lysterfield one time, when the Sight (27.5) was in the shop. Got 11 PRs that day that still haven't been beaten. That's a pretty convincing argument re: speed, for me.
Sight's still more fun, though!
I had a different result. Went from a 29 100mm bike to a 275 130mm bike and instantly beat almost all of my PRs that weren't flat firetrail.
I'm sure wheelsize has a bearing on things, but it's much less than the industry would have you believe. Advances in suspension, and to a lesser degree geometry, have had more impact IMHO. We can pedal more efficiently, chattery trail slows us down less, and climbing technical trails is able to be done easier and with more traction.
Wheelsize is a thing, but I'd put money on not being able to tell whether I had a 275, 26, or 29 if I was riding the same bike and wasn't able to look at the wheel.
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