Aero drag goes as the square of wind velocity and power is proportional to the drag times the ground velocity. So in a 20 mph headwind the required power is proportional to 30^2 x 10 or 9000. That's the same as if you were riding 20.8 mph with no wind at all (20.8^3 = 9000). Of course that's assuming that aero drag was the only force working against you (no rolling resistance, gradient, mech. inefficiency) so in practice your equivalent speed without wind would be a little less - maybe 19 mph or so.

If you can't go close to 20 mph using the same level of effort then I do question whether the wind was really that strong. Keep in mind that the wind speed down near the ground is usually significantly reduced. So your wheels, legs, and lower part of the bike may have been seeing a wind speed of less than 20 mph.

Note to 10 Wheels: The topic of this question would seem to relate equally to all types of bicycles and is certainly not specific to folders. Just because someone lists a folder under his list of bikes shouldn't prohibit him from posing reasonable questions to other subforums where they apply. Furthermore, sreten lists 'Folder, Road Bike' under his list of bicycles - I read that as indicating both a folder and a regular road bike (but again, the relevant question should be about the topic of the thread, not the bicycle owned by the poster).