A large part of the B-Twin not holding speed will be due to tyres & hubs and to some extent being less aero.
I've had a Propel and now have my Emonda. I can/could ride either just as fast.
The biggest thing I think is getting into an aero position. We're both riding Emonda's, whilst the frame tubing is not aero - the geo does tend (at least imo) to put you into an aero position, which is key.
I see some riders out on the roads & in Sportives etc riding aero bikes upright like Mary Poppins, may as well pop a basket on the front.




so do punctures.
