I'm not an experienced cyclist by any means, so can only comment on a comparison of two bikes, one being my son's mountain bike and my own racer. Son's mountain bike. Once round the block and yours truly has rubber legs. In a nutshell, it kills me. On my racer I can cycle for quite a few miles (getting better all the time ). The racer is a much lighter bike than my son's mountain bike - and it is very noticeable. In my case, it is not due to body weight as I weigh 6 1/2 stone (about 91 lbs - you could say I'm a flea weight). I had one of those luggage racks on the back of my bike and took it off last week as I didn't like or use it anyway. Even taking that off I found made a difference to my bike. I actually wonder what it would be like to ride a bike even lighter than the one I've got. Also just discovered there is a fairly new dedicated bike shop in town. Guess where I'm going tomorrow