Sure, I'm not going to talk about the ridiculous amount of time every standalone level requires, again (more than a couple minutes for a test almost-empty level, with an SSD, I wanna cry. Don't say "in PP it's shorter", because I reverted to 05/12 release and Public Preview is too full of bugs so it's unusable).
By the way, I seriously hope GG will come out REALLY soon with an update that REALLY works. If not, my project is stuck in a limbo and I'm full of frustration.

However. I tried to put a loading music that fits the game, by editing "loading.lua".
Ok. Fine. But the music barely starts, then stops after a few seconds because Loading screen freezes (and appears as not responding for Windows) until it's completed.
Any workaround to avoid this, and let the music play?

And yet you are
J/K

If your having issues with the Loading.lua then your better off asking in the scripts section

Not in detail, indeed.

Anyway, is it what happens to everyone (I mean the loading screen freezing and "not responding" unitil loading is done)?
Not only related to music in the script, but in every loading session.