My new iMac seems to struggle with flicking through photos on iPhoto. These are 20MP RAW files, but after flicking through about 7 or 8 I get the 'beach ball' for a few seconds and then it takes another few seconds before it is ready to go again. The iMac is a 3.4 I7/3TB Fusion/680M config and I have tried this with the iPhoto library on both the internal Fusion drive and a USB3.0 external. I have had the opportunity of trying two identical machines side by side and neither could do this. I am sure I didn't have the same problem with by 2008 iMac with a Core 2 Duo!