I used to commute on my dahon mu sl folder (9 and a bit miles one way). It didn't feel slow at all, and I got a real kick out of beating people in proper biking gear on proper bikes, while in my jeans with a backpack on a tiny little bike

In the end I bought a 29er and put big apple tires on it though, because the folder was not comfortable over rough bikepaths. My commute is largely on slabs of concrete with tar expansion joints poking out between them, and the constant slamming tired me out. On the 29er it is much more comfortable, and I am 1-2mph faster as a result. However, this speed difference is almost entirely due to comfort I'm sure, if my commute were on billiard smooth roads it'd be not much different since my mtb is hardly "aero" .