Immediately after updating to iOS 4, some users may notice that Exchange ActiveSync Mail, Contacts, or Calendars do not sync, or sync very slowly. In addition, some Exchange Server administrators may notice their servers running slowly.

When an iOS version update occurs, you are replacing the Operating System (OS) on the device with a new OS. For example, updating the iPhone 3GS from iOS v3.x to iOS v4.x replaces version 3 of the Operating System with version 4. This update often forces the client to do a complete mailbox refresh from the server meaning they entire Inbox/Task/Calendar is re-downloaded after the update.

Apple will be releasing a new update to iOS 4 at some point that will resolve this.

Apple has published an Apple KB article (http://support.apple.com/kb/TS3398) with a link to a configuration file that can be downloaded to the iOS 4 devices through E-mail or with the Safari browser on the iPhone/iPad. This configuration file will raise the timeout value from 30 seconds to 4 minutes.  The file is a basic XML file that can be edited using a text editor if needed.  The default value is 4 minutes, but can be raised in circumstances where 4 minutes is not enough.  Note: 4 minutes is enough for the vast majority of people.