Any folder is quick with the right engine..however if you have a cheap 6 speed folder with a 46 teeth chainwheel to a 14 to 28 freewheel no matter what you do you will never be very quick..All down to gearing ,weight,tyres,front profile..etc etc..Dahon P18 or Tern 24 speed or even a Dahon Uno will be fast..
One guy on here has a 24 speed Mezzo which can give a road bike a run for its money..No doubt some fit Brompton riders can..