For devices using iOS 6.1, Apple suggests this software update: http://support.apple.com/kb/DL1639, now available on the Apple site for those who wish to use it.
Original announcement, February 12:
A known issue between Exchange 2010 server and devices using iOS 6.1 can cause performance issues affecting both McGill’s Exchange servers and individual iPads and iPhones using iOS 6.1.
Problems reported for individual iOS 6.1 users at McGill and other institutions include: excessive cellular data usage, potentially causing battery drain and possibly higher cell phone bills, as well as performance issues in Outlook calendar.
The cause and temporary fix are explained in this article from Apple support: http://support.apple.com/kb/TS4532. As stated, an upcoming software update from Apple should resolve the problem permanently.