Adding "/prefetch:x" doesn't generally make things faster - and it doesn't en/disable the Windows Prefetcher, as the urban legends have it.
It was included for big multi-purpose applications, where operating in different "modes" cause different codepaths to be taken and thus needs data from different sections in the executable. It's explained here
, and the specific example given is media player... playing a CD is different from playing a DVD is different from playing an MP3 is different from playing an AVI file, et cetera.
I dunno if it's used at all for Vista and Win7, where the XP prefetcher was replaced in favor of the more aggressive SuperFetch.
Be very very wary when reading those various performance tweak sites - most of it is based on misinformation and urband legends, and usually the authors are either passing on bad information verbatim, or are making misguided guessed based on something they saw in the registry and don't understand.