It's not that the car is slow. The problem is that every time you take it out, a mass of people appears with a lot of questions. Or they'll just stand in front of it to take their pictures.
On the highway, you'll need lots of extra time to change lanes. Other drivers will drive up next to you and gawk at your little car. So you'll slow down. Then they'll slow down. Then you'll speed up. And they'll speed up. You'll wave your arm to tell them to slow down or speed up so you can get over before you miss your exit. They'll smile and wave back.
After a few months, the novelty will probably wear off. By then everyone will have seen a Smart ForTwo on the road at least once, and they'll stop pointing at you with their mouths open.
This may also be the end of the biggest reason you bought the car in the first place. There are other good reasons, but there are serious problems too, and some of them are quite bad.