I've done an Audax SR (200km, 300km, 400km and 600km rides) using my heavy Thorn Raven Tour.
The last 2 "Around the World" world records have been set using Rohloffs.
As others have mentioned, put it in 11th and compare bikes. In 11th gear it will be at least as efficient as a Derailleur, probably more.
If your bike is slower in 11th gear then I suggest that the problem is tyres or the fit of the bike (or you)
Fit some different tyres. Panaracer Paselas 26x1.5 or less. Pump them to the max recomended pressure and try again. If this isn't faster, in 11th gear, than a MTB with MTB tyres then check your brakes aren't binding.
Note. "Faster" is subjective. My Raven Tour feels very slow because it's silent and smooth. However, I have a timed 15 mile circuit I use occasionally. This loop proves to me that my Raven Tour is 20 seconds slower, over 51 minutes, than my "fast Tourer". Wind-direction or temperature on the day can account for 20 seconds easily so, statistically, I would say there's no noticeable difference.
I suggest you do a timed circuit on different bikes.
The answer to your original question is "Yes" but it depends on many factors.
Is a Raven Sport Tour, with slick road tyres, slower then an MTB with MTB tyres? "No".