"But it's using reflection, so it must be slow"

Well, not that slow apparently.

The lesson:

Don’t be afraid of powerful tools.

You can use reflection right, gain the power, while not losing too much performance.

Quoting from nhusers mailing list:

How much you be scare about the use of reflection in NH if 1.000.000 of access to get & set to a field mean 0.2seconds ?– Fabio Maulo