The reason calendar appointments take longer than mail migrations are Domino and Exchange use a totally different system for Calendaring in the back end. One example; Domino you can make a recurring appointment and change it almost any number of ways, where Exchange has somewhat limited ways you can set up recurring appointments.
To compensate for this, the tool has a lot more processing to do to make the Domino calendar appointments work with Exchange. The mail is basically exported to RTF format and injected into Exchange.
