Folding bikes can be quick, but the term quick is relative. I don't ride a folder for it's speed, I ride one so I can take it with me on the bus or train (I don't have a car). I can get around quickly enough on a folder, but for very long rides at a consistently high speed, I will use my old road bike. I used to do almost all of my regular riding on my folder, until it was stolen last month. Now that I have to do my commuting on my road bike, I find that it takes nearly half as much time as it did on my Brompton. But, on the other hand, I can't take my road bike on the train or subway, so I am not going out to the countryside as much as I was.

If you are going to get a folder, speed should not be the main priority. You should choose a bike which is reliable, reasonably light, folds quickly and easily, and has a wide enough range of gearing for the type of riding you will be doing.