In this question it is suggested that the planet Krypton is somewhere between 27 and 2000 light years from Earth. And in this question it is suggested that it took Superman between 2 and 20,000 years to get to Earth. (But prior to the events in the movie Superman Returns Superman took a 5 year leave of absence to go see the remains of Krypton, and then return to Earth). And some of the answers to this question suggest that Superman travelled along with Kryptonite to earth.
So if Superman was launched in a spaceship as a baby (and he left at such a rate as to be clear of the exploding planet), and the remnants of Krypton were just accelerated by the exploding planet, how did the Kryptonite get to Earth in the same time frame as Superman?
Was Superman's "baby ship" non-powered and only just stayed ahead of the debris? But if the ship traveled at supra light speeds, how did the Kryptonite achieve approximately the same velocity?