There's been no mention of the energy sapping rear suspension block on the Brompton. Every time you put pressure on the cranks a tiny bit of your energy is wasted compressing the suspension block. The harder you push, the more energy is lost to the bumper. I would replace it with a solid nylon block. I can't find one after a quick search, but it should be a simple job to make one from a suitable diameter of nylon rod for someone with reasonable DIY skills by just drilling a hole through the centre of the rod. I should add that it's many years since I owned a Brompton and the design of the block may have changed now which may mean a more complex solution is required than I'm suggesting here.