The only reasonable advantage to defragmentation is efficient reallocation of FAT references of most used files in any given HDs.
Knowing this, any "slowdowns" in accessibitity is more about integral detection of regular groups swapped into activities WHEN and from where exactly. If your "programs" seek too many chunks at once, the lag time can impact performance.
Thus, bigger files (Booting the OS, comes to mind) can load faster if called for in optimal sequencing. Smaller files (by quantity) strains on the laser needle pointer, too.
Clear up the unused junk. Maintain integrity of core assets, defrag once a month or less... these simply are a function of maintenance.
Proper empty space re-organization on a drive is MUCH more important as the most commonly used files must be easier to reach. NTFS solved that by indexing sufficiently. Defrag on the other hand, simply keeps track of your continual usage trying to determine what files matter most; from this process, a suggestion is offered to you -- split or fuse.
In nanoseconds terms, you boot reasonably fast(er) or slighty out a synch.
Anything under 5% is fair game, i heard.