I clued into this around 10 or 12 years ago. While it was absolutely true that my wider tires were slower, at any pressure, it was because I'd buy the most inexpensive durable wide tires that I could find. By wide I mean 32's. Even when I wanted more supple 32's they couldn't be found, unless you were willing to pay 5-10 times the price for them.
I came to realize that if width and pressure make a difference - and I still don't doubt that they do- it's much less difference than the makeup up tire makes.