They are faster because they are lighter than anything else out there, 5.5 grains per inch. For most bows this will be below the 5 grain per pound minimum total arrow weight required to not void the warranty.

Also, from what I have heard most people's experiences with them, they cause the bow to be louder (from shooting a lighter weight arrow, heavier arrows absorb more energy of the shot making it quieter) and also very brittle and prone to splintering and cracking.