MacBook Pro is very slow.

Easy one. Change the hard drive/IR cable out and use the 2012 models cable (see link). It gets damaged both by the rough texture where the hard drive sits on the cable and rubs and also on the last edge as it climbs the multiple steps. The bottom actually impact on it. I put pads on either sid of the cable to prevent this. I also use slick electric tape on that surface where the hard drive sits. If this has gone on a while it may also have damaged your hard drives data. You may need to back it up an do a low level (write zeros) re-formar of the drive the install a system and use migration assistant to move your data back. Might as well go to an SSD or SSHD for improved speed.

MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable MacBook Pro 13" Unibody Late 2011 Hard Drive Cable Replacement