Physical resources on the client machine like CPU, memory, disk speed, and network connection speed
Physical resources on the Microsoft® Exchange Server (or IMAP server) like CPU, memory, disk speed, and network connection speed, along with how well you’ve tuned your Microsoft® Exchange Server performance
The overall speed of your network and your connection to external networks
The density of traffic outside your network
You can increase the amount of data you process by increasing the number of users you process simultaneously on each client machine, and the number of client machines you use.
In current testing, using Microsoft® Windows Server 2003 and later, you can optimally process 20-50 users at a time on a client machine, depending on the client machine’s configuration.
Google Apps Migration for Microsoft
Exchange Administration Guide