Interesting, OT. Are you using iCloud Photo Library?
My iCloud Photo Library (45.645 items) needs roughly five minutes to open in Power Photos, whether I have Photos running or not. The smaller libraries not syncing with iCloud will open much faster.
Another reason for the long startup time required may be the many smart albums I have. In my main library I have 170 smart albums that need populating in Power Photos by retrieving the photos from Photos, since they cannot be directly read from the Photos Library.
I noticed that it takes a long time for smart albums to update in Power Photos when I modify a smart album. And even Photos is slow to update smart albums and showing the busy indicator for a long time when I create more smart albums.
