The software doesn't determine "fast" or "slow" data transfers. Users may adjust their file sharing speeds but one FS program can't be "faster" than another. All file sharing software does is keep a current centralized database of files available for sharing. The transfer of the files is between the two parties and does not go through the software's servers. The original Napster did in fact allow the transfers to pass through their servers which led to it's demise. When files do not actually pass through the file sharing servers it prevents them from being successfully sued/shut down by the RIAA.