I don't think it's the Rohloff. I have been using hub gears (as well as derailleurs) for decades and, when bikes and tyres are similar, I don't notice huge differences in efficiency (which I estimate by comparing average speeds) between most hub gears and decent derailleur setups.
The Nomad is a very heavy bike. I wanted one after my 2011 tour, but was persuaded by Andy Blance at SJS/Thorn to get the lighter and slightly less robust Raven Tour instead, as I don't intend to do round the world touring.
But once you add all the luggage for a long tour the extra weight of the Nomad shouldn't make a huge difference.
For me, the two biggest factors affecting riding efficiency are the riding position, where I find drop bars more efficient than most other bar setups because of the better aerodynamics, and tyres. Very robust tyres suitable for round the world touring are much harder to pedal than the same size tyre in a lightweight but less robust model. So if your Nomad has expedition tyres and flat bars, on normal roads I would expect you would have to ride slower and use lower gears than on your Ridgeback Panorama.
Until I bought my Thorn, my long-distance tourer was an old mountain bike, with a frame about the same weight as the Nomad, but not designed specifically for load carrying.
Before using it on my last long tour in 2011 I modified this bike with drop handlebars, the lightest fat tyres I thought I could get away with (Marathon Supreme, but the Marathon Dureme recommended by Julian are tougher and only slightly slower), and changed the (derailleur) gearing to be as low as possible. Timing before and after rides, the bar and tyre changes combined made about a 10% improvement in average speed.
I never bothered when I was younger, but in 2011 I also trained specifically, starting about 6 months before my tour and doing 50 km loops about twice a week as fast as I could, with occasional longer rides when time permitted.
To ride the heavy bike (39 kg with luggage at the start of the trip) up the mountain passes in the Pyrenees and Picos d'Europa, I had an 18" lowest gear, which I used often.
On my current Thorn Raven Tour I specified 38x16, which gives a lowest gear of 17".