Are you using a Solid State Drive (SSD)? SSD drives do not work the same way that hard drives work. When you delete a file, that space is not automatically used by a newer file. When you increase the size of a file, the additional space needed may be in a non-contiguous location, and that could slow things down. On an SSD, collecting unused space and optimizing files is called garbage collection. Older SSD drives might not have come with a garbage collection routine (Crucial SSD’s do). The older Mac versions needed to use a separate TRIM utility to do garbage collection. Again, with macOS X High Sierra and the new APFS file system, there is no need to be concerned about performing garbage collection on an SSD because as long as the computer is on and not busy, SSD garbage collection is done in the background, and it works even with older SSD drives but ONLY if the SSD has been reformatted as APFS. If you upgraded to High Sierra or purchased a new mac with High Sierra, the reformatting of the internal drive is done for you (except for some third party SSD’s). So you just need to be aware that any external drives may not be formatted as APFS, and you have to do that yourself. There are many additional features of APFS that work in the background for you, and it’s worth knowing something about those features - even if you don’t care to know.
John R Carter Sr