There is one trick. While the library is being built switch between different tabs (e.g. Rules > Publishing > Rules). Usually, this helps to have the library marked as ready much faster. Let me know if that works for you?
Thank you very much for your answer. So if I understand correctly, the library gets built relatively quickly but it's the notification of the "library built" that is slow to get displayed?
I built the library several times very early this morning (Eastern timezone) and it was way much faster than when I do it in the afternoon. I guess the speed is also a function of how many concurrent users are building libraries.
Thanks again for your answer, I'll check your trick this afternoon.