Originally Posted by FBinNY

IME, weight differences are almost all about hills. I do some (lightly) loaded touring, and utility riding besides commuting and sport riding. I've come to the conclusion that every 15 pounds or so is one gear step down on the same hills, so a loss of about 10% or so climbing speed. Otherwise on level ground, once the bike is rolling I couldn't care even if I was hauling 60 pounds.

One added caveat, if you commute in a built up area, with lots of stop and go, or speed changes, then the weight penalty is that much higher because you're working harder to come back to speed after every stop.

I wasn't being facetious earlier when I suggested hauling potatoes. The easiest and most reliable way to know how weight will affect your time is to add the weight and see what happens.