My MacBook has become too slow

I’m suspecting your HD SATA cable is failing you. Here’s the IFIXIT guide to replace it: MacBook Pro 13" Unibody Mid 2012ハードドライブケーブルの交換 and here’s the needed part: MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable.

This is a common issue in this series as the uppercase aluminum is rough and wears the cable where it crosses over. I recommend you apply some electricians tape where the cable rests to help protect the cable from be abraded. Also don’t over bend the cable as you don’t need crisp folds.

Once you get the cable replaced you’ll want to do a fresh backup as you’ll want to reformat and install a fresh copy of OS onto the drive.