I am syncing, on several Macs and several user accounts, information such as Calender, Mail Rules, Bookmarks, etc. After recently updating to Safari 4.01 and OS X 10.5.7, all of a sudden I had all very bad Safari performance -- including constant freeze situations. After being ready to delete the impacted user, I realized that there were some issues with the database.sqlite3 file in the Library » PubSub » Database folder. The PubSub folder is used for tracking RSS feeds, and it seems my problem resided there.

After deleting the database.sqlite file, and reseting the Mobile Me information, the freezes and crashes stopped. Hope this helps others, too.

Safari4の調子が悪い場合に試してみると良いかも。