Joking aside, it's actually a good question and not that easy to answer. Exactly what is causing it to be slower?

1. Weight. Roads are most often not exactly level, so a heavier bike will be slightly slower on slight imperceptible uphills.
2. Position. A more upright position will have more wind resistance and slow you down.
3. Wheels. Smaller wheels have more rolling resistance, due to more rubber movement relative to big tyres, and more revolutions on wheel bearings, and generally slower tyres.

So a lighter, more aero, with sskinny wheeled bike will be faster than a Brompton. But it won't be as convenient.