External boot drive on Mac computer, suddenly going slow.   
Two days ago I noticed a sudden slow down of my Mac computer.  It boots from an external drive and when I re-tested that drive and compared the results with those of last week, the data speeds had halved.  I checked the SMART status and there was no degradation of those parameters and the Mac was not reporting any issues.  I swapped the housing for a known good one and tried some known good cables.  I ran the Mac Disk utility "First Aid" and even I reinstalled the OS from a backup but still no improvement.  Apart from the speed issue the drive runs without issue or error.  The only inconvenience is that I will have to replace the drive a few months earlier than planned. It is 10 years old with power on hours of 75,000, so I have had a great run out of it but I am just curious to know what could have made it run slow so suddenly and without any errors etc.  Suggestions will be appreciated.  The drive is a 1TB hybrid Seagate ST1000DX001 SSHD.