The thing that makes hard drives slower is fragmentation. Regular disk defragmentation can significantly improve your computer's speed, especially when launching programs and working with larger files. It occurs when the file system cannot or will not allocate enough contiguous space to store a complete file as a unit, but instead puts parts of it in gaps between existing files - usually those gaps exist because they formerly held a file that the file system has subsequently deleted.
You can schedule regular disk defrag during downtime.
