CPU, that's often called just "processor", is an abbreviation for Central Processing Unit. That's the core of each computer or web server, since it performs all of the calculations and logical input/output operations. Although the performance of a site or an app depends on other things as well, like the amount of physical memory or the connection of the web server, the speed at which a particular processor operates determines how fast a program shall be executed. Later-generation processors have a number of cores which can drastically improve their overall power and efficiency, because each and every core can take on a number of processes separately or a number of cores can take on a single process which needs a considerable processing power. Because every core operates at a certain speed, this architecture can be seen as a number of independent processors working together.