Got the MTB all donned properly with its new rigid fork at the weekend. It's dropped 1.25kg off the front end! Overall the bike is probably slightly lighter than the Kaff (better steel, lighter fork, better/lighter wheels, no guards etc etc). So with a spare Carradice bracket attached, and dry roads, popped onto the MTB for the commute after a relaxed 6 or 7 miles on Saturday.
Maybe it was the more upright riding position into the hefty headwind. Maybe it was the flat pedals making uphills interesting as I lifted my feet when having to honk. Maybe it was the slightly wider 26" slicks compared to 700s. But by eck that was hard work.
Oh, and kaputnik, you're right, the bearings in the headset are completely shot - need to get replacement parts (naturally it's a lovely sealed MTB unit and not just replacement cages).