CPU, that is sometimes called just "processor", is an abbreviation for Central Processing Unit. That's the core of every personal computer or server, since it performs all of the calculations and logical input/output functions. Although the performance of a site or an app is determined by other things as well, including the amount of physical memory or the connectivity of the hosting server, the speed at which a particular processor runs determines how soon a program will be executed. Later-generation processors have multiple cores that can considerably improve their overall power and efficiency, as each and every core can control several processes separately or several cores can deal with a single process which requires a considerable processing power. Because every single core operates at a specific speed, this architecture can be seen as several separate processors cooperating.