The same rules apply in folders as they do in 700c bikes. The most important aspect of a bike with respect to speed is the tire, tube, and rim. The tire must be low rolling resistance, normally the narrow, high pressure tires 100-120 will have lower rolling resistance.

The tire/rim/tube weight is also the most important as they are rotating and moving forward.

Gearing is important. Many folding bikes are designed to be ridden with heavy wide tires and wheels so do not have the high gears needed to ride 20-24 mph.

Front chainrings need to be larger than most road bikes to compensate for the smaller wheels.

Since most folders have smaller wheels (20") either 406 or 451.

There does not seem to be a difference in rolling resistance of 20" vs 700c bikes if the tires are comparable. There is an advantage in drafting to the 20" wheel as the draft is better the closer you are to the rider in front of you. With 20" wheels you are closer and so is the guy behind you.

The negative is good high pressure 20" tires are not as easy to find as 700c and will often have to be special ordered.

Folding bike frames are often a little heavier due to hinges and binders and having to beef up the material around the hinge.

Often the more expensive folders will be lighter and faster. Usually if the bike comes with 451 wheels it is set up to be a fast road bike.