A recent Apple support note offers help for the times when iOS 4 slows way, way down during syncs with Exchange ActiveSync Mail, Contacts, and Calendars. The problem may also bring performance problems for Exchange servers.
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.
Apple says the work around for this issue is a new configuration profile that increases the time the iPhones will wait for the Exchange Server to respond to a sync request.
For best results, the profile should be installed on as many iOS 4 devices at your company as possible.
Updated Sunday, July 11: This fix appears to have mixed results, according to the MacWindows blog.