In T3 skynet was a peer to peer program that infected all the computers on earth and used that processing power to launch the attacks and do what it needed to do.
Whats the Petaflop rating of all pcs , laptops , iphones , smart phones , gps , ds , psp , consoles and so on and so forth in the world ?
Of course when T1 was first made back in the 80s TFlops was huge. it was the 80s after all.
It does spread out by the end of the film but when its first activated in T3, presumably on some central server, its quoted as operating at around 550 TFLOPs I think. So it must have become self aware at that level before making the decision to spread out across the web.