Many people take one look at the small, awkward folding bikes and say no to even trying them. Transportation that tiny can't possibly be efficient, in their minds. In reality, folding bikes, especially those with smaller-than-normal wheels, can be quite speedy. The size of the tires and the bike, plus what you add to it, will all affect the overall speed.
Folding bikes often have more compact, smaller frames and tires to let the bike fold up into a smaller package. The smaller equipment also lightens the bike, making it easier to carry. Without these, there's not much point to owning a folding bike for most people because they likely got one to store it in a small apartment or take it with them on a bus or train, or into the office because of a lack of bike racks. The tires' size will affect how fast the bike can go, as will anything else you add to the bike. Folding bikes can have full-size tires, usually 24 or 26 inches, and look like a regular bike. They can also have -- and most models you see will be in this range -- 16- or 20-inch tires. One model even has super-small tires at 8 inches in diameter. Smaller tires make the bike look awkward, with the seat and handlebar posts sticking out above the rest of the frame.
A folding bike is not only easier to carry because of its reduced weight, but easier to move through pedaling, too. If the bike is lighter, that's less you have to try to shove around by pedaling furiously. Peter Yuskauskas of NYCeWheels also notes the smaller profile of the bike means there's less wind resistance and air turbulence from the spokes, allowing the bike to get up to a fast speed very quickly. Wind resistance basically pushes against a bike, slowing it down. This may seem like an imperceptible difference to you, but over a long ride, that extra pressure can add up, equaling a longer time on the bike and possibly legs that are slightly more sore.
The gears play a major role in getting a folding bike to go at higher speeds. Yuskauskas writes that the gears on the bike actually change the effective wheel size. The gears on a folding bike will actually be higher than equivalent gears on a regular-size bike. Normally when you shift up to a higher gear, it's because you're on a section of road where you can pedal fast, such as a section going downhill. The shift makes it tougher to pedal but also makes it easier to move quickly -- it's as if each pedal is now connected to a larger wheel, covering more ground. Place higher gears on a folding bike, and now you can really make the bike move.
Folding bikes have a better response time to your movements. This can make the bike seem unstable, especially if you have started pedaling quickly. Don't try to go too fast on the bike until you have the hang of just how the bike acts. Smaller tires may also seem rickety to those used to big mountain bike tires. In 2009, "The Guardian" had two men try switching bikes. One usually rode a folding bike with small wheels and the other a full-size hybrid. The man who tried the folding bike noted, "All my body movements were exaggerated with often near-fatal consequences. As I took a hand off the handlebars to signal ahead of a junction, the sudden upset to the bike's equilibrium sent me veering dangerously towards a parked car or oncoming traffic." Again, practice riding slowly until you get used to stabilizing the bike.