So is it 26 or 27.5"

In case you didn’t realise I like them. My other two. Both xl

I did a local red route circuit a couple of weeks ago in bone dry conditions.11 mins 31 secs. The fastest I've done in years. This was on my 27.5" Boardman FS Pro. I took my Whyte PRST4 up but felt a bit jaded. I thought I may as well still have a go at a fast lap. 11 mins 12 seconds. That, to me, is a statistically significant result. The 12 year older bike is 19 seconds quicker. The downhills were a bit more daunting and it was skipping about in places where the Boardman would have gripped. But since you spend most of your lap climbing the lower tyre drag and magic, intuitive linkage front suspension don't rob you of nearly as much energy.
Someone needs to bring back linkage front forks. BMW have done it for motorcycles so why not? 150mm of travel would suit me, matched at the rear. Wouldn't mind 27.5" wheels, curiously, or even 29ers. I'd chose a narrow tyre on the rear in dry conditions and use an unfashionably high pressure but reckon I'd get round my test track quicker than on any other bike.

Re:

Less than 3% quicker.

Looking through my Strava for similar timed segments shows a much wider spread over just one of my 26" bikes, let alone over all of my bikes.

Are you a racing athlete, if so then yes 3% is significant as you'll know what your best is.
Not one, then it is easily day to day variation, ground conditions, tyre pressures, wind direction, gps differences, when you last ate..