[math-fun] fastest computer ? (single operation).

Hello, As you may know, the fastest computer is currently a japanese machine called K, which can do 10 petaflops (?) or something like that. These machines are made with ordinary components at the base like Intel processors or Opteron + many GPU like Nvidia. This is all very impressive but I am wondering, what is the fastest single operation computer on this planet ? A souped-up Intel machine running overclocked at 4 ghz ? Is there somebody that knows the answer to that question ? In other words, if someone wants to run let's say the Mersenne test for a prime which is not yet implemented in parallel then on which machine would that algorithm run the fastest ? Best regards and ha ve a nice evening. Simon Plouffe