I added another machine to my Time Capsule. It took 14:20 to back up 48.4 GB, ~3.4 GB/hour — and this is the one machine in the house that is connecting at 1000BASE-T. This is the slowest backup yet. Admittedly other machines were backing up during part of this time, but no large amounts.
I think there are occasional performance issues, and have filed bug 5801685 with Apple.