On the sort of bicycle that one usually finds an IGH, the hub is not the limiting factor. My admittedly limited IGH experience is with S/A 3 speed hubs, both old and new, and a J.C. Higgins with an Austrian clone. The latter was my intro to the joy of internal gears. Anyway, my present commuter is a 1974 Columbia Sports 3. Heavy, yet kind of flexy. I like the ride from the long seat stays and mild steel, though. But, the frame probably sucks up more energy than the SunRace SRF-3 hub. And, it is running Schwalbe Marathion Plus tires in 42-590. Heavy! But, with Chain-L on the chain, a wipe down when getting the salt off the wheels and underpinnings is all that is needed. I did flush the hub and converted to ATF from the factory grease at about 2 years and 2500 miles. At that point, I replaced my 20 tooth cog with a 23, and replaced the chain as well. This seems like pretty low maintenance to me. And, what the bike looses in speed, I save in time not flossing gears, adjusting derailleurs, etc.
There are varying degrees of losses in derailleur systems, depending on the size of the gears. Probably cross chaining, small ring to a small cog, is the worst possible efficiency. But, whether you slept well, had your second cup of coffee, etc. all will relate to speed more than the sort of drivetrain, given similar bike geometries, tires and other variables.