I don't know if it's just an option in 2.2 or if it's there on 2.1 and I haven't tried it myself, but there are some options in menu, settings, voice input & output, text to speech settings. There's a speech rate setting in there. The only thing is that it will affect your speech settings in everything... But I'd think if Maps/Nav speaks too quickly for you, everything else on the phone does as well.
I'd think that if you checked always use my settings and then set it up how you wanted, it should work. I don't believe there's any kind of speech rate setting in Maps/Nav. Like I said, I haven't had need to try it, but it seems like it should work.