I found it somewhere in the discussions, but cannot find any official bug reported to that topic. So let’s have it here - for borderless it’s expected to have similar experience while in full screen mode, but without any minimisation whenever we click outside of the game. That’s how it works (or should work), thus the fact that it still minimises the game (so behaving exactly the same way as with full screen) is a bug and provides no real profit from using borderless mode.
And indeed the loading times are so painful, that it’s necessary to have an option to look at something else while the game is loading and still see if it loaded already or not (plus it sometimes crashes when you try to open the game that is under loading while minimised).