It opens and plays music in less than 3 seconds, on my x86 NixOS box.

I wonder if the macOS build has a bunch of static libraries it has to load first.

Static libraries don't need to be "loaded first". They're part of the executable. That's why they are called static.

I had to test because I didn't recollect ever experiencing such issues with Spotify.

And no, the problems you described are not there.

On my Windows 11 laptop it takes about 2sec to start and basically any song starts in less than a second.

Maybe it's Apple making Spotify crappy on Mac to promote iTunes?

The app has gotten consistently slower, buggier, with worse UX for over a decade. In all that time, I can’t recall a single new feature I’ve actually found useful.

The app was done in 2011. They could make a massive improvement across the board with a single git reset --hard command.