Weight increases inertia and gravitational drag; it resists acceleration, and resists climbing. For a ~200lb bike+rider, a kilogram on the frameset will increase both of these resistances by around 1%. Which is to say, if you add a kilogram to your frame, and you go do a several-thousand-foot climb out in the mountains, don't be surprised if the time you spend climbing increases by the better part of a minute.
When riding steady on flat ground, it doesn't really matter.