I bought some cheap Kojaks for barakta's trike before she went electric. At the right pressure they were significantly faster, but that was coming from Marathon Plus. The main difference was the much smoother ride and improved (on skog-free tarmac) grip. I can believe that the suspension benefits of a more supple tyre on a small-wheeled (20") semi-rigid tricycle were a significant part of the improvement, and I wouldn't be surprised if that also applies to Bromptons.
(Now she's got a motor and rolling resistance is mostly irrelevant, I've gone to two Big Apples on the front and a Marathon Plus on the back. That gives a reasonably smooth ride, works off-road and minimises the chances of having to faff about removing the motor wheel to fix a puncture.)