When folks talk about performance differences between for example AES, Serpent, Twofish what exactly are they referring to? Do performance differences only refer to how fast a hard drive can be encrypted? Or once encrypted does it refer to how fast encrypted files are decrypted to be put into RAM? I read that Serpent is fastest on hardware but slower on software. What does this mean? Is encrypting a harddrive with Serpent a hardware or software implementation? Any insight/explanations on this would be much appreciated.