MacBook Pro 2012 not performing after changing SSD.

Hello, all. I have recently swapped out my hard-drive to an SSD. The first day it ran like an absolute dream but since then it has been awful. Sometimes when powering it on it comes up with the circle with a line through it (I believe this indicates and OS issue?). I used internet recovery initially and then upgraded to High Sierra. If i get it past the circle with a line through it it runs extremely slow and cannot actually perform any tasks. I understand it’s an old machine but would just like to know if there’s anything more I can try before deeming it a lost cause.

Thanks in advance!