Several uses have reported slow scrolling and other performance issues in iPhoto ’11, along with an issue in which the application briefly or indefinitely stalls.
Delete cache. Try deleting the following folder com.apple.iPhoto from:
Rebuilt iPhoto library. Hold down the Option and Command keys while you launch iPhoto and select all options except for “Rebuild the iPhoto Library Database from Automatic Backup”
Switch thumbnail size. Press the “3″ key to switch to medium sizes for thumbnails. This can increase scrolling smoothness and speed.
Increase RAM. Albeit a less-than-ideal fix for this specific issue, several users have reported dramatically improved performance from iPhoto ’11 after adding more RAM.
Downgrade to iPhoto ’09 (8.0). Failing all else, you can downgrade to iPhoto ’09.