The greatest bottleneck for traditional spinning-platter hard disk drives (HDD) is the time taken for the drive head to spin around to find the section of the disk it needs, known as ‘seek latency’, or ‘seek delay’. Typically this is around 3ms on modern hard drives, and this builds up noticeably when reading or writing many small chunks in different places on the disk platter.
This is why defragmenting the files on a Windows machine will improve its speed, for example: aligning files to avoid disk seeks means less time spent waiting for the disk to rotate into position.
Solid-State Drives (SSD) do not have any moving parts and so avoid this physical bottleneck. As a result, SSDs are able to perform many times faster for non-sequential operations than a traditional hard disk.
Use of SSD can improve performance for many applications, but we recommend considering them especially for IO-intensive applications such as database use, which may involve many small non-sequential reads or writes.