I was surprised that painting the second coat took the same amount of time as the first (even though I did not repaint all the cracks this time). I am gonna say that the full-strength paint was harder to apply and spread less quickly than the thinned first coat, which seems to be the only reasonable explanation.
For the curious, the floor is roughly 250 square feet, and took 3 hours per coat. All of it was done by hand with a 3" brush (both coats).
But it's done now. I went through a bit more than 3/4 of the can, so I'll need to pick up another one to do the other two rooms (when I get to them).