I have this same issue, it started about 2 or 3 weeks ago. Photo stream had been working fine for over a year, then out of nowhere iPhoto went unresponsive. I had narrowed it down to the photo stream by looking at the activity monitor and saw that photo stream was killing my CPU. Once I turned photo stream off in settings my iPhoto worked again. However, I still don't have a fix for having Photo Stream turned on and a functional iPhoto.
Have you solved this problem on your own?
FYI, I have a new Macbook Pro with Retina so I shouldn't have any hardware issues.