After opening iWeb '08 the app asks for a keychain upgrade. After allow, the message is still there and then everything slows down. It takes quote a while before finally opens the app. In the meantime most other apps do not move an inch.
This morning installed iWeb '08 2.0.1. Result is that the Mac is not slowing down and that it keeps working, but still the problem exists. It turns out that the Keychain gets completely empty. After a restart this is solved.