Why is my MacBook taking 30 second for each action? (unresponsive)

I still using MBP Mid-2012, and it works just fine for more than 10 years until a couple of days ago. I have upgraded my storage to 500Gb SSD and also my RAM to 8GB. In the last 9 years, I almost always turned off my Mac every time I put it in my rucksack (I know it shouldn't be a problem, it is just a habit). Last week, I forgot to turn off my Mac while I commute and when I got back home and open the computer, it started to become unresponsive.

The cursor is still responsive and has no problem. However, it needs at least 20-30 seconds to respond to a single click or hover. Even to type something it takes 10-15 seconds until it appears on the screen.

I tried my best to do some possible actions I can find online to solve this. Turn off unneeded Login Items, uninstalled apps, cleaned junk, cleaned malware, and check+ and repaired the storage via Utility Disk until I decided to wipe out my SSD. It takes forever to do internet OS recovery* and install a new Catalina and I keep bumping into the same problem. I decided to reinstall the OS that came with the computer (Mountain Lion), and it's still the same. I decided to reinstall Catalina again, however, It's not getting better.

I've opened the machine, and checked for any possible physical damage to the hardware but can't find any. In addition, I installed a new pair of RAM. I run Apple Hardware Test and got the reference code ADP000.

So, probably not the OS and not the hardware? I'm literally clueless at the moment and desperate for any solution. What should I do now?

Thank you!

*I don't have a bootable OS installer