Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Speed up iPhoto
Performance of your iPhoto depends on various factors, such as age and power of your machine, available hard drive space, amount of RAM, size of your library etc. If you feel that your iPhoto is running slow, here are some tips to make it faster.
1. Add more RAM
1 GB of RAM is recommended for smooth performance of iPhoto. However, the more RAM you add, the better.
2. Use keyboard shortcuts
Keyboard shortcuts will save you lots of time when working with iPhoto. The complete list of shortcuts is available under Help, Keyboard Shortcuts.
3. Speed-up scrolling thumbnails
Making the iPhoto thumbnails smaller will save you minutes of scrolling through their list.
4. Rebuild iPhoto library
Rebuilding the library will significantly decrease the launch time of the app. To rebuild the library, hold the Option and Command keys and open iPhoto again. In the Rebuild Photo Library dialog, select the Rebuild all of the photos’ thumbnails and the Examine and repair iPhoto Library file permissions options. Then relaunch iPhoto and see the difference!