A few weeks ago I dug out my old cirka '91 partly rusty roadbike. I wanted to use it for commuting to work. Anyway, after the first day I was ready to throw it out the window, my time to work had increased by about 6% and the reference was a same conditions ride on my 8,5 kilo hardtail, but with knobbies.
I looked at the hubs for drag, spinning them in my hand, then looked to the bottom bracket for drag, none... very strange until I actually tried spinning the wheels on the bike... FRRRrrrrrr......stop. DOOOOH! Quick adjustment of the brakes and now its running very well again. It's ~11 kilos, but what the hell.