Computers are only machines, and often they fail due to human error. In fact, they always fail due to human error. After all, humans built them. That is why slow computers simply need a bit of a refurbishment rather than new hardware. When writing to a spinning disk like a hard disk drive, it is done in segments. Over time you will delete some things and as a result, there will be fragments all over the disk of memory with small space in between.
This increases the read and write time whilst the computer searches for the data it needs and finds large enough spaces to write to. This can be stopped with a utility called a defragmenter which will shuffle all your data together and re-organize your disk. It can even increase writeable capacity now that there is no tiny space left un-written to!