While my regular commuter was out of commission, I had to suffice with a friend's bike: a 1980s Fuji Boulevard, which is sort of like a super heavy mountain bike with cruiser bars. Nothing has been done with it since it left the factory in Taiwan. After I got used to the new machine and its friction shifters (and the friction of the brakes rubbing on the steel wheels, etc etc), it became fairly comfortable.
In fact, my commuting time in the fair weather I was fortunate to have for the 4 days I used it was THE SAME as on my usual commuter: 40 min for 10 urban miles.
Granted, my commuter is not built to be *fast*, but rather dependable, fun, and perform well 365 days a year. But, it is modern and well-maintained, and certainly more efficient than the bike I borrowed.
I'd guess that I wasn't doing the same max MPH, but with hitting traffic lights and everything else, the difference turned out to be negligible. Funny, and sobering.