I used to do a 20 mile (round-trip) commute on it, when I changed offices to one that was 35-40 miles round trip, i upgraded to a road bike. The Brompton is wonderful, but it is much more exhausting to ride than a light road bike....if you want to go fast, be prepared to get really sore, if you want to maintain a steady heart rate and level of effort, be prepared to go really slow.

If you are concerned about efficiency, the kind that lets you do long distances everyday at a good speed, I'd say go for a road bike.