A system can become slow or unresponsive after migrating images from version 1.0 to 1.1 RAW decoding in Aperture. If you look at Activity Monitor, it shows Aperture using a large amount of CPU resources.
After you update images from 1.0 to 1.1 decoding, Aperture must regenerate each image thumbnail. If you update a large number of images at once, this can cause a noticeable slowdown. To avoid this issue, migrate images in relatively small batches (only those needed for a particular session), or migrate a large batch when you plan to be away from the computer for a while.