Well, that depends on what you're measuring. Certainly the couple needs to be at home on the bike before attempting to tour on it. However my wife climbs twice as fast on our tandem as she could on her single. I obviously climb more slowly on the tandem but I don't care - I never wait for her and she's happy. Two people of equal ability climb exactly as fast on a tandem as on their singles. I once came across a couple on loaded singles, climbing Rainy Pass. The desire to kill was written all over her face. That's what tandems remedy.

Otherwise, it's all about getting the load down. We carry, for two of us, about half the load that many single tourists carry. As a result we climb about one cog lower loaded than we do unloaded. Loaded, our cruise is about 1/2 mph slower than unloaded. The panniers are all but unnoticeable in a headwind, due to the famous tandem draft.

Nancy_sv is currently in Bolivia on a tandem with her family. Erickson Tandems do supported tandem tours in the Alps every year.