engedib:I had this a few months ago with a MacBook Pro 2011, put a new SSD into it, same results. Dreadfully slow.
Then found out that the cable which connects the drive to the motherboard is broken. It is really screwed design, the cable is a very thin one and it bends quite sharp a few times. 😫
After getting a cable as well, it was running million times faster.
https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Unibody+Mid+2012+Hard+Drive+Cable+Replacement/10379
I see a lot like this, it's an awful design. I'd suggest popping your drive out and booting it from a USB enclosure or adapter to see if the issue remains.



