I have the same problem – stuck on "preparing library" for 20 minutes at same 48% on progress bar. I have a large number of photos, and a lot of them are TIFFs. Ironically, I started Photos so I could see if I could turn off its automatic start whenever I plug in my camera (I use different software, and now am really glad I do). But Photos software failed in 3 ways:
* no access to Preferences (grayed out) until the library is prepared
* Preparing library was either stuck or frozen, with no indication of what was going on
* No way to back out once Preparing Library is stuck; window close box grayed out, and so is Photos "Quit" command.
I lack the patience of many commenters here, and since my purpose was not to use Photos (and with this experience it definitely isn't) I used Force Quit to exit Photos. This was an unfortunate and disappointing encounter with this software.
[using OS 10.11.4 on a late 2009 Mac Mini with 8 GB RAM]