Immediately after updating to iOS 4.0, 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.
Update your device to iOS 4.0.1 or later. If you previously installed the configuration profile in this article, remove it by tapping Settings > General > Profile > EAS Default Task Timeout > Remove. Contact your IT Administrator if you are unable to remove the configuration profile yourself.
If iOS 4.0.1 (or later) is not available for your device, proceed with the following:
To work around this issue, users should install a configuration profile from Apple that increases the amount of time the iOS 4 device will wait for the Exchange Server to respond to its sync requests. For best results, the profile should be installed on as many iOS 4 devices at your institution as possible.
Use Safari on your iOS 4 device to download the signed configuration profile(http://km.support.apple.com/library/APPLE/APPLECARE_ALLGEOS/TS3398/SignedDefaultEASTaskTimeout.mobileconfig). It may be easiest to read this article with your iPhone or iPod touch and tap the download link there, but you may also email the attachment to another email account on your iOS 4 device, and tap the attachment to open it. Do not attempt to open the profile using iPhone Configuration Utility.
Tap Install to install the profile.
Tap Install Now, and enter your passcode if prompted.
Tap Done to complete the installation, then power off your device and power it back on.
Wait for your Exchange data to finish syncing.
After installation, the profile will be listed under Settings > General > Profile.