Why is my laptop suddenly so slow?

Surprisingly you have a bad HD SATA cable! You’ll need to replace it MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable and here’s the needed guide MacBook Pro 13" Unibody 中期2012 硬盘线缆替换. You’ll want to place a strip of electricians tape on the uppercase where the cable crosses Your Hard Drive Cable Is A Ticking Time Bomb.

You also need to be careful with the cable when you put it in. A common issue is people crease the cable at the needed bend points which damages it! You need to create a radius bend to do this you’ll need something to help you form it like a BIC ballpoint pen ink straw (empty). Use it to help form the bend at the corners.

The last issue is fixing the buildup damage on your drive. Backup your drive using TimeMachine and make a USB thumb drive bootable OS installer so you can reformat the drive and install a fresh copy of the macOS. Then using the migration assistant restore your stuff from the backup. Done!

Here’s something worth looking at: if you open Utilities > Activity Monitor and look at the processes that are going on in the background, do you see anything using a lot of CPU (over 100%)?

I’ve run into issues like this and found it to be anything from a corrupted OS to a bad thermal sensor to a bad battery. Anything that the system interprets as a behavior that puts the hardware at risk will engage a process that throttles the CPU and causes very slow behavior…