The clock cycle, or the amount of time between two oscillator pulses, determines the speed of a CPU. The oscillator, or oscillator crystal, is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electric signal with a precise frequency.
Piezoelectricity is the electric charge that accumulates in certain solid materials (such as crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress. The word piezoelectricity means, literally, electricity resulting from pressure. The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in crystalline materials, the effect is a reversible process, meaning that just as the internal generation of electrical charge can result from an applied mechanical force, an applied electrical field can result in the generation of internal mechanical strain.
Anyway, the crystal oscillator produces a fixed sine wave that makes up the frequency reference signal. Electronic circuitry then translates that into a square wave at the same frequency for digital electronics applications (or, in using a CPU multiplier, some fixed multiple of the crystal frequency reference frequency). The clock distribution network inside the CPU then carries that clock signal to all the parts that need it.
The time between the two oscillator pulses, or the clock speed, is measured in MHz or GHz. For example, a 5GHz processor performs 5,000,000,000 clock cycles per second.
For some time now, the clock speed achievable by crystal oscillators every year has increased by roughly 2 times the top speed of the previous year, allowing for processors to run faster and faster.
Electrical engineers are constantly trying to figure out how to combat the heat generated by the oscillator once it passes certain speeds and gate delays. Liquid nitrogen and liquid helium have both been utilized in an attempt to combat the heat.
That said, the clock speed achievable by a CPU is not the only determinant of the processing speed of that CPU. For an example of this, take the fact that some processors can send one instruction per clock pulse while others can send more than one.
An instruction is an order that a computer program gives to a computer processor. The order is composed of a set of binary codes that together describe a physical operation the computer is to perform as well as information regarding particular storage areas that may contain data relevant to that order.
Processors that can send multiple instructions per clock pulse will work faster at a given clock speed than processers that can perform only one instruction per clock pulse.
Bus speed is another factor outside of clock speed that affects processing speeds. A bus is a circuit between two parts of a motherboard. The width of the circuit determines how much data that circuit can handle at once, with wider circuits being able to handle more. Bus speed refers to how much data can be moved across the bus simultaneously.
Excessive clock speed can actually destabilize a processor that doesn’t have matching upgraded components.