For max speed, I haven't been past about 140rpm, which is about 31mph with 74.2 gear inches (and a good downhill).
I actually don't do a lot of riding in heavy traffic, so I don't know what would feel too fast for me in traffic. I like to cruise between 15 and 20mph on the level when I have plenty of space to myself. If I'm feeling competitive for some reason, I've been known to fly at 25 on the level at times.
10mph feels really slow to me.