Clock speed of the processor
The CPU executes 1 instruction per ‘tick’ of the CPU clock. A typical CPU clock speed is 2GHz (2 billion ticks per second!). The faster the clock speed the more instructions per second the CPU can process.
Number of cores
Most modern CPUs are multi-core, meaning effectively they have more than one processor per CPU. For each extra core the maximum number of instructions increased
e.g. 1.2 GHz Quad-core
4 x 1.2 = 4.8 GHz total.
Size & Speed of the CPU Cache
The CPU contains its own on board area of RAM-like volatile memory called the Cache (the is where the registers are stored). The larger the cache, the more data that can be stored without accessing storage external to the CPU (e.g. the RAM or HDD storage). The faster the cache speed the faster data can be sent to the processor.
Type of architecture
Some architecture types such as RISC architectures support techniques such as pipelining, which can increase the effective number of executed instructions per second.
External Factors that affect CPU performance
RAM size and speed
The faster the RAM memory that quicker data can be sent to / retrieved from the CPU, which means less idle time for the CPU. The larger the RAM the more data can be loaded from the slower secondary storage( such as HDD / SSD) and so also less idle time for the CPU.
HDD / SSD speed
The CPU spends a lot of time waiting for data to be loaded from the secondary storage. The faster the read/write speed of these devices, the less time the CPU will be sitting IDLE.
GPU Speed
The CPU often offloads intensive graphics processing tasks to the GPU and then waits for the GPU to process the tasks. The faster the GPU speed, the less time the CPU sits idle.
Software Optimisation
Some software is optimised to take advantage of certain CPU features, such as software that supports parallel processing