There's a lot of hard information somewhere on this forum, if only one could find it.
There's no need to be too greedy with the Big Apples: you don't need the full 60mm of the biggest ones. According to Herr Kalkoff, who built the Pedersen bikes in Germany, the 50mm gives you 90% of the advantages and no hassle about finding and setting up SKS P65 mudguards to within a millimetre. There's also a 55mm Big Apple, which on the example I rode was indistinguisable from the 60mm on my bike.
The 50mm is a fraction under 2in, and the 55mm is usually reckoned as 2-1/4in wide, the relevance being that there was conversation here once about which of those would fit on at least some Thorns.
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Far from slowing you down, these tyres make you feel powerful, like stepping up to a V12 Mercedes from a Mondeo. And it isn't just psychology either. Once these tyres are going, they just keep rolling. One of those really, really competent technical universities in Germany made comparative tests and found that of all the tyres they tested, the Big Apples had the lowest rolling resistance. This doesn't surprise me in the least as I'm an old auto-racer, and for two reasons: the wider the tyre is, the better its grip patch, which is a tiny thing, is shaped, so that the energy transfer has more frictional interface to work into, and the slicker the tyre is, the less hysteresis in the rubber sapping the energy from your legs. (It was amusing watching Jobst Brandt, a revered Porsche engineer -- not to mention a cutting edge designer of bicycle components, trying to explain to the cyclists on a so-called technical bicycle group why slick tyres grip the road better, even in the wet.)
As I say, it's not psychosomatic at all. Those tyres are faster for the same input than narrower tyres and especially treaded narrower tyres. I used to demonstrate that on the flat but even now occasionally someone on a road bike tries to stay with me on the downhill and it never works out well for them, mainly because their inadequate tyres on those bad surfaces put them all over the place. I'm not only faster, I'm safer, because nothing intimidates my fat tackies and I swerve for nothing at speed but ride through it, including potholes that will bend a thin rim behind a high-pressure narrow tyre, and because my bike is set up to understeer, whereas the pilote of the road bike may, without him even knowing it, have, and most likely has, been sold a twitchy bike, just waiting to oversteer out of control. I don't think cyclists appreciate, and I'm certainly not going to try and explain something so counterintuitive (it's bad enough trying to explain why fat low-pressure tyres are superior in almost any application but racing), how often it is the width of their tyres that throws them off, with unnecessarily steep frame geometries being the next culprit. I'm on another conference where "a wider tyre" means going all the way up to 23mm... I just chuckle and don't get involved because it isn't worth the flame wars with people who last had their minds in gear about the time the Peugeot 10-speed first appeared.
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Interesting subject you've chosen. The short answer is, as you've divined: by the seat of my pants. A lot of this theory not only contradicts long-time cycling wisdom passed through generations, raising fiercely protective attitudes, but is counterintuitive as well. If you look at the token treads on Big Apples, you will understand what I mean: the designer knows those tyres would be most efficient if they were as bald as an egg, but the marketing director know that he'll sell fewer tyres without at least a token tread. The sell, or sold last time I looked, a special treadless "racing" model of the Big Apple, at a premium of course.
