Exchange 2010 introduces new client throttling features which can have a direct impact on migration planning. The feature is designed to prevent some clients from consuming excessive server resources. As with cell phone bandwidth usage, usage distribution is often skewed, with a small percentage of users consuming 95% of resources. Unfortunately, as a provider of cloud-based migration, our usage patterns are quite different from what you could expect from a single client responding to emails. After all, our technology is designed to push as much data as possible in the shortest amount of time possible to reduce the impact of migrating data.
From a migration perspective, two behaviors directly affect migration using Exchange Web Service APIs. The first behavior is concurrent connection limits per login. We’ve found that this setting applies even when logging on to different mailboxes using administrative access. As a result, trying to login to 1000 mailboxes concurrently using a single set of admin credentials may fail. One solution is to use the following commandlet to enable higher limits: http://technet.microsoft.com/en-us/library/dd298094.aspx. Another solution is to specify connection limits on your MigrationWiz connector (see advanced properties).
The other setting which can affect migration performance is CPU utilization limits. Essentially, this setting determines how much CPU can be used to process Exchange Web Service requests vs. other things. The good news is that some email providers have started segregating servers used to process Exchange Web Services vs. those used to process OWA HTTP or MAPI RPC requests. Machines dedicated to processing EWS requests can be allowed to reach higher CPU utilization without risking interference. Also note that MigrationWiz automatically detects and adapts the speed at which it migrates data based on hints returned by the Exchange server, and this in a client/server cooperative manner.