MacBook Pro Mid-2012 SSD suddenly extremely slow I have a severe issue with an MacBook Pro Mid-2012 (serial number is C1MK9X6ZDTY3; it is a MacBook Pro 13-Inch "Core i5" 2.5 Mid-2012). In 2017, the RAM was upgraded. Also the internal HD was replaced with an SSD.

Recently, the MacBook's OS was updated to Catalina because some other software required the upgrade. The update essentially worked just fine and I could work with the laptop. However, a few days ago the system slowed down significantly up to a point where it's not usable anymore. Right now, directly after rebooting, I can hardly open any apps. The colored spinning wheel appears immediately and the machine more or less freezes. Also I cannot properly open Finder, etc. 

I tried to connect the SSD externally via USB to another Mac. The behavior is the same.

I did some checks on the SSD: 


Disk Utility says: https://imgur.com/a/ljvuHHt
The log when trying to do "first aid" on the disk says the
following: https://imgur.com/a/FgXVoDu


As you can see, there are some errors. Could they be related to the
    OS update?


I also installed DriveDx: https://imgur.com/a/BhBA6aa Looks good,
if you ask me.


Can you help me identifying the issue here?
I've also struggled with the same Macbook type and encountered a similar problem. Replaced the HDD with a SSD and it worked for almost two years, until the Mac became really slow.
Looks like the SATA cable is broken and needs replacement. See these ifixit instructions for the replacement.
The SSD I've used was a SanDisk SDSSDA-480G which is really slow now, even when I put it in an USB enclosure on another Mac. Writing is now limited to 2MB/s, while reading is still faster (around 250MB/s). I've verified with a different SSD and HDD that the enclosure isn't the problem for those low speeds. Looks like the disk is also damaged as side effect from the SATA cable.
I've replaced cable and SSD with new components and the same machine is now fast again.