Computer speed depends on the performance capabilities of its various components, the most important of which are its central processing unit, random access memory, its long-term data storage drive and its graphics processing unit. Together these devices save the computer's data, send it to areas where it's available for processing, and perform calculations on it based on the software commands that result from user input. The faster these devices do their work, the faster the flow of data and the greater the computer's overall speed.
For most applications, computer speed boils down to how fast the CPU manipulates information. This depends primarily on its clock rate, amount of cores, cache memory and bus speed. Clock rate is the frequency at which the CPU operates. The higher this value, the more times each second the CPU carries out software instructions. The CPU's cores are independent processing units. The more of these the unit has, the more tasks it can work on at the same time. Cache memory sits on the CPU itself, supplying it with the most vital information to its needs. The larger the cache, the less often the CPU has to retrieve information from other slower sources. Bus speed determines how quickly the CPU can get information from the RAM, its primary source of data.
Computer speed also depends on how fast the CPU gets the information it needs. Most of this information comes from RAM. RAM receives requests for information, fetches it from long-term storage and supplies it to the CPU. It also stores the computer's operating system and open applications. The more RAM a computer has, the more applications and files it can work with simultaneously. RAM also comes in different speeds and types. The faster the speed, the more times per second the RAM accesses the cells that store its data. Type determines how many of these cells RAM can access at a given time. For example, DDR3 RAM can access eight cells simultaneously, allowing it to alter eight bits with one access as opposed to four, in the case of DDR2. Thus, the faster the RAM and the more recent its type, the faster it can get data to the CPU.
Long-term storage drives save your information when your computer is off. The effect the drive has on computer speed lies in how fast it reads and writes information. The faster it does this, the sooner it can send data to RAM, and the faster the CPU can do its work. Most computers use hard drives consisting of disks that store data magnetically and "heads" that read and write the data on the disks. The higher a hard drive's RPMs, the faster the disks spin and the quicker the drive can prepare and send information. Flash storage drives do not have this issue. They have no moving parts and can thus access memory much more quickly. These improve computer speed, especially in those with limited RAM.
The GPU is a different type of processor from the CPU. It handles the lion's share of the calculations demanded by video editing, rendering, and graphics-intensive applications. These programs require more instructions to be processed simultaneously than most other types of programs. When these programs are in use, the GPU is vital to computer speed. Its speed depends on its clock rate, its number of cores -- sometimes called stream processors or CUDA cores -- and the speed, amount and type of its built-in memory. The higher these values the faster the computer will be when you're playing 3D games, creating 3D animation, editing images or watching video.