Here's a comparison i found years ago...
I had a Raleigh Chimera, chro-mo frame and forks, heavy, Sora 8 speed, rather a heavy bike, but its was never high quality, but didnt cost a lot to buy.  I had years of good service from that bike, covered thousands of miles...a bit uncomfortable and jarring on rough roads, but overall..ok.
I then got a 9 speed campag equipped Bianchi, the pre C2C model with the straight top bar, alloy frame, alu/carbon fork.  It was like a Rolls Royce compared to the Raleigh.
 
But, about one year later, it occured to me that if i considered my time to do my usual 40 mile loop on both bikes....there was virtually no difference, 5 minutes maybe.
 
Give me a light bike any day, it won't neccessarily make you faster, but you'll probably enjoy the ride more, feel more comfortable.