It was the surface of the keys. The effect is one of extra drag on any sliding or even rotating movement. Movements between black keys that depend on smooth, easy, dry sliding are very difficult. Even hymn tunes become difficult. Currently humidity is 62% at 20 C but I think it was worse a few days ago.
My favourite theory is instantaneous molecular condensation at the point of finger contact. I favour this idea because I can't see any other explanation for the persistence of the problem on an instrument which is stable at room temperature. If the keys were actually wet then they would be MORE slippery I would have thought. Clammy, sticky, slow: feels like a mixture of dry and wet. Or, that theory could be rubbish.