Why does copying programs on Mac OS to the Programs system folder take a long time?

Question

macOS

Hello. I didn’t seem to notice this before, but when installing programs, at the stage of moving the program to the Programs system folder, some kind of long copying takes place. Yes, I moved the Downloads folder to the hard drive (the system is on a solid-state drive), and the copying actually happens from the HDD to the SSD. But not 10-15 seconds for a 50mb file. Now I just tried to copy the image of the installed program (dmg) from the HDD to the SSD - instantly, without a progress bar. The SSD is not even half full. What could be the problem?
There is a hypothesis, but not sure. After opening the image, a folder with files is already copied, and there can be many of them. This multiple copying can also slow down the process.
Why is this happening?
OS - High Sierra (10.13.3).