http://support.apple.com/kb/TS3398
The following is a proposed fix for the extended delay users are seeing on IOS4 for their 3gs and 4 iPhones.
From the article:
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.
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 company as possible.
To install the profile:
- Use Safari on your iOS 4 device to oc?this.s_oc(e):true' href="http://km.support.apple.com/library/APPLE/APPLECARE_ALLGEOS/TS3398/DefaultEASTaskTimeout.mobileconfig">download
the configuration profile. 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.
- Tap Install to install the profile, and enter your passcode if
prompted.
- A warning message will appear because the profile is not signed.
Tap Install Now.
- 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