3.5 mph. Approximately. (Did you know 78% of all statistics are made up on the spot?)

Explanation in generalities -- you can quibble with every assertion I'm about to make, but on average, I think this is pretty descriptive. Mechanical losses are pretty close to negligible for a well maintained bicycle. So that leaves two contributors to the difference in speed, tires and aerodynamics.

Road bike tires are going to have less rolling resistance than the average hybrid tire. Hybrid tires, although they're all over the place because there's such a range of bikes called "hybrids," will, on average, be heavier, run at lower pressure, and have more tread resistance (knobs and such).

Your average road bike rider, riding on the hoods, will be somewhat more aerodynamic than your average hybrid rider, who will tend more toward a sit up and beg posture

Finally, although your question neglects it, your average road bike rider will be more performance oriented -- younger and harder charging -- that your average hybrid rider. If you put the same rider on both bikes, had that rider in the same position (which would probably involved bending the elbows more on the hybrid), and put similar tires on both bikes inflated to similar pressures, the first bike tested would be fastest just because the rider would be fresher -- whichever bike went first.