Not much slower than on my steel road bike. But riding conditions tend to be different, the Brommie is used in more trafficy situations, and less open road. I use ridewithgps and it gives me a "moving average" figure, which (as I have grumbled many times before) is totally meaningless as an absolute measure of speed but it is useful for relative comparisons between rides. And the answer is "a bit slower than the steelie ... But sometimes a bit faster, if the Brompton ride is flat, and the other one is hilly"