Why is my HDD (HardDiskDrive) so slow  ?         

This is super common this problem., and the reasons are manifold.
A.K.A.  rust or spinners and the like, (it has a motor and has slow moving heads)

In order of likely to happen. (age is #1 ) Buying wrong drive is #2
  1. It is a RUST drive and not any SSD or M.2, solid  state drive, and was slow new and is slow today, what you see is what you get ! (sorry) The benchmarks public are here.
  2. Is the drive too full (the cure is to delete unused files or move them to backup media, DVD-R or external drive enclosure devices for USB)
  3. The HDD fails S.M.A.R.T test, and is a bad HDD if it fails, and can fail and be slow for no more extra spare sectors and is just worn out. 5years life is par, if the HDD auto-repair feature is dead,so be the drive slow
  4. The wrong HDD drive was bought , is slow, the best drive is a SeaGate  ST6000DM001 , and is this drive does. (227MB/S average as seen below)  MB= MegaBYTES
  5. Using the wrong tests to measure speed,  I use ATTO or HDtach.exe., CrystalDiskMark, or the great, userbench mark, or my licensed PASSMARK.
  6. Your OS is corrupted, or infected,  or have too many upgrade engines running in the startup folders of Windows. (seen PCs with 25 upgrade engines, and Adobe? pain) (in task manager startup turn them all off)
  7. Did you test the HDD on a 2nd PC for speed and full SMART Tests? (even as a 2nd drive or in a drive dock or drive enclosure)? The hope here is 2nd PC is not infected, ok?
  8. Did you make sure the CPU utilization is low while speed testing? (par is 2% never 100%) see red arrow below?
  9. Did you try more that one HDD bench mark program to be sure you are not being lied to.?
  10. In Left field (Buzzzz) your PC is 10 years and you stuffed on W10, and the OEM maker of this PC does not support W10, and the driver for your South Bridge chip is now wrong and yes, slow. (cure: talk to intel.com)
The intel South Bridge support is only here and ONLY WHAT YOU SEE HERE .( supports only series 6 UP) (Q6* ,H6* etc) up. Like Q67. back to 2011.
Running the wrong SATA port drivers,  can make the HDD run slow.
Some PC will show a tad slower scores if AHCI  SATA mode  is not enabled in BIOS (10% less) (and the horror's of changing that)

This program app. is old 2007? but is still good, SATA is after all still SATA 2001 to now.  (must be installed in compatiblity mode !)   This is high water mark test.

Only 54GB of 6TB my data backups used on drive (no where near full)
ATTO, newest version. 4.01.0f1 date released 08/14/2019
Hits 200 easy. (USB-C boxed) I'm proving the USB-C chip in the housing can in fact be fast with the fastest single HDD made by Seagate .

Old drives 10 years old even brand new, run 2 to 4 times slower, and is how old drives worked, unless they are 15,000 RPM drives or in a RAID array.
The older they are the slower they are, even day 1 new, first SATA drives started in 2001.
There are 1000's of models of HDD,  for speed or for life span and even for video servers. (the worse HDD are 2.5" seen in vast laptops, pure junk IMO)

As you can see the evolution of HDD technology is not dead.

HDtach. ( I like their method of testing and graphics) all downloads tested by me with Virustotal.com , never do I forget that step.
Yes it may be to simplistic. (I is one of 10 bench programs in my KIT)
Right click the installed application, hit properties, go to compatibility, and run the program in compatibility mode for windows XP SP3
When here, only click this link seen there, or you will get other junk you do not want like PC Health BS.

Test condidtions< W10,64bit,  v1909+ , Asus Z270prime, tested in USB-3.1 port and USB-C and SATA 3, both  and with my new SEAGATE 6TB drive, ST6000DM001
The Public benchmarks on my HDD is here.
Hint, yours is there too, if you just look it up.
The fastest drives are told here.

version 1.  12-12-2019