How fast your computer performs a task is determined by what you run out of first. Old computers may run out of memory. If you are copying large video files, this will be limited by the speed of the disk.
When you are playing a video game or encoding a TV show so it can play on your iPhone, then you might run out of CPU power. That is probably the only task that ordinary users do where a faster CPU will help. If you juggle a lot of large files, a second disk may be the best upgrade.
In downtown New Haven, CT where I-91 meets I-95, the “Q Bridge” crosses the harbor area. It must be one of the hottest attractions in southern New England, because every morning and afternoon cars line up for miles to cross it. It defines the rush hour commute, and nothing that you do to the other roads or exits in West Haven or East Haven will materially speed things up.
Inside your computer there is an electronic version of the Q Bridge. Depending on the application, some component will become the choke point, and all the data bytes will line up waiting to get through. But while the real Q Bridge never changes, the PC choke point moves as you change use.
A Porche and a Yugo get caught in the backup at the same point in West Haven. Twenty minutes later they cross the bridge at the same time. It doesn’t do any good to spend a lot of money on a fast car and a big engine if the limiting factor is traffic moving five miles an hour. Yet customers often select a server with a fast CPU, without first considering what the bottleneck will be.
If you play video games or edit video, then the speed of your computer depends on the speed of the CPU. If you record TV shows on your computer and then edit out undesirable material periodically inserted into the program, processing may be 10 times faster if you read from one disk and write to another than if you use a single disk. The performance of a database is typically determined by the amount of memory you have. Copying files from one machine to another depends on the network speed.