RE: I dont understand how carbons can be faster....
Depends on the carbon arrow you buy. If I were to pic an aluminum shaft for my set up it would be a 2415 which weighs in at 564 +- grains for a finished arrow. Now take a Carbon express CX300 with the same components and it only weighs 400 grains finished.
THat's how a carbon arrow is faster. The carbon arrows typically weigh less grains per inch than aluminum shafts do with similarly spined shafts.