Complex sentence: LeMond's 54.545 km/h average speed for the stage 21 time trial was the fastest in Tour history; since then, only the 1994 prologue and David Zabriskie's 2005 time trial performance have been faster.
Split into simpler sentences: LeMond's 54.545 km/h average speed for the stage 21 time trial was the fastest ever riden in Tour history. Since then only the 1994 prologue and David Zabriskie's 2005 time trial performance have been faster.