Smartphone laggy and slow: when a factory reset is ineffective
I am passing by to post a possible solution to those who have similar issues.
Although I cannot say it is definitive/long-term (I did it only yesterday) the device is really much more responsive now, and the hangings and crashes seemed to disappear.
The steps were (without deep details):
In my computer:

Downloaded a stock ROM from a reliable source (mind the exact version);
Downloaded Odin v3.13;
Installed Samsung USB drivers;

In my Samsung S6:

Wipe Cache (recover mode), then reboot.
Started it in Download Mode.

After that, plug the USB cable between the phone and the PC, and flash the firmware using Odin.
I followed these steps. It may help someone too: https://www.youtube.com/watch?v=UEnm5TU5KMw
In my case, I needed a factory reset after the flashing because the new FW did not erase the data. Now everything seems fine. I wish I had discovered that before buying a new smartphone. However, no problem... I'll give this one to my father. :-)
Hope someone finds this useful.
See you guys!