iMac late 2012 running slowly. What could be the problem?

Question

macOS

Good day,
In general, the problem is that the iMac late 2012 is insanely slow.
All you need to know:

• Programs open slowly
  
• Finder is slow (copying, deleting files, etc.);
  
• The operating system loads a normal time, not for long;
  
• Checking the disk did not reveal any errors (even for checking bad sectors);
  
• Although the computer is quite old, the previous owner, in addition to watching movies and listening to music, did not load the hard drive with various heavy operations;
  
• According to the state of the computer 10 out of 10, it was not dropped, not beaten;
  
• I'm not talking about working in heavy programs, but such hardware should guarantee the smoothness and speed of operation of just an operating system and light programs, which is not the case in practice;
  
• A month ago, everything worked fine, after one day, when the operating system stopped loading at 100 percent load when the computer was turned on. After a clean installation with formatting the disk and installing the OS without third-party media, and the native OS, it all started.
  

Characteristics: The

operating system is fresh, immediately after reinstallation (done a couple of times), so that possible options for "garbage" in the system are rejected. In fact, apart from a couple of triplets of programs, there is nothing on the computer.
There is a guess that the weak speed of the hard drive is to blame. Will installing an operating system on an external SSD drive help fix the slow operation of the OS, in terms of not performance (I'm not a fool), but the overall speed of loading software and operating systems?

Answer 1

Almost always, it is the HDD that is to blame for the slow operation of older computers. There may not be bad sectors, but if there are many slow sectors, then everything will work slowly.
If the screw is to blame, then of course it will help, but why is it external?