The first thing to do is attempt to open your OWA URL from an external network from a system that is not joined to your domain. If OWA is loaded in a timely manner, check your server resources:
- Check the server CPU usage.
- Check the memory usage.
- Check the disk I/O usage using performance monitor.
- Check for over-utilization of the network.
The most common problem encountered is the overutilization of network. This means that you are attempting to migrate too many mailboxes simultaneously, and do not have enough bandwidth to execute them. Refer to Introduction: Throttling Bandwidth. Once you have a better understanding of network utilization, configure the correct number of mailboxes you can migrate simultaneously.