Busy cursor and Mac OS too slow even moving to SSD

Lets try dissecting the problem a bit here:

• First AHT shows no errors (from your screenshot) so your logic board is most likely OK

I would start by booting up on an external drive (USB thumb drive) while the USB drive won't be as fast as your internal, does the system respond better? If it does, I suspect your SATA cable is bad and/or the systems firmware was not up to date. Here's the part info for the drive cable: MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable. Apple had a bad run of these cables we have had to replace quite a few. Here's the IFIXIT guide to follow to install it: MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement. I would start there more often than not this is the problem. Here's the Apple T/N to check & update the firmware: EFI and SMC firmware updates for Intel-based Macs. You will need to run this from your external drive! If you don't you could corrupt your firmware making your laptop a great boat anchor which I'm sure you don't want!

The next possibility is the SSD drive and systems SATA speed is not workable together. Can you give us the exact model of your SSD so we can review the spec sheet.

If the SSD & system SATA speed are compatible: Still using the external drive. Run Disk Utility to check the internal drive. Did it pass? If it did you may need to free up some space on your SSD. I strongly recommend leaving 1/4 to 1/3 of the drive left free (the smaller the drive the larger amount needs to be set aside). That way the system has enough work space. If it failed your SSD is bad.