I have considered this question over the years, as I like the 26" platform but, in real world application, do find 700c to roll faster. If only it weren't so.
What I think is that it goes beyond tire size to construction considerations.
Until recently, it was hard to get a 26" size tire designed for speed. Most come out of the off road or touring genre where durability is prioritized over speed. The same goes for wheelsets. Again, until recently, most 26" wheels were build for toughness rather than speed while 700c has a longer history (and trickle down effect) related to racing like lower spoke counts, aero design, light weight materials etc... And, lastly the gearing/geometry of 26" bikes usually hasn't been optimized for speed (unlike many 700c road bikes).
Add all those factors up and 700c has been historically faster than 26".
I'd like to see two similar bikes, using similarly designed wheelsets, with 26" and 700c tires go toe to toe. A part of me would be cheering for the underdog
