Good day,
In General, the problem is that insanely slow iMac late 2012.
All you need to know:
- Slowly open the programs;
- Slow Finder (copying, deleting files etc);
- The operating system is loaded the normal time, not long;
- Disk check did not reveal any errors (even to check bad sectors);
- Though the computer is quite old, the former owner, in addition to watching movies and listening to music of various heavy operations the hard disk is not loaded;
- As computer 10 out of 10, it did not drop, did not beat;
- I'm not talking about extreme programs, but this iron is smooth and the speed just OS and light programs must ensure that is not in practice;
- A month ago everything worked fine, after one day when the OS will not load to 100 percent load when you turn on the computer. After a clean installation with formatting the drive and installing the OS without third-party native, and native OSes all began.
Features:

Operating system fresh, immediately after resetting (was done a couple of times), so possible options on the "trash" in the system are rejected. In fact, except for a couple of triples of programs on the computer there is nothing.
I have a hunch, because of a weak hard disk speed. Will installing the OS on an external SSD drive to fix the slow OS in terms of performance (I'm not a fool), and the total download speed of software and OSes?