Running El Capitan. iMac has plenty of memory. I am creating calendars with photos and am having to force quit because iPhoto 1.2 not responding. All updates current. Happening once or twice every session.
iMac, OS X El Capitan (10.11.1)
Try repairing the library according to the instructions IN this Apple document: Repair Your Library.
Note: If you use iCloud Photo Library with Photos, this action will cause Photos to upload all your pictures again (from Fix missing or incorrect thumbnails in Photos - Apple Support).
Needing to force quit iPhoto 1.2 too often
