As far as torsional, lateral and vertical stiffness are concerned, a metal bicycle frame is a very stiff, undamped spring. But a CF frame could also include a lot of damping, often designed into the layup e.g. Bianchi CV. Modern CF frames are so stiff in torsion and lateral flex as to make any energy losses when sprinting insignificant - especially among similar modern bikes you may be considering back-to-back. Vertical compliance is what people really notice on modern bikes and even that is less significant now we are moving toward wider, low pressure tyres.