If multiple migrations are running at the same time, the first configuration that is migrated will automatically use up all the available threads. Unfortunately, this leads to the items within the other migrations having to wait to start.
You can use the "Simultaneous Migrations per Configuration Limit" option to limit the number of items that can be launched for that configuration. This means that each configuration will be limited to the amount of threads it can consume per server, freeing up threads to start other migration configurations at the same time.
To modify this setting, you can go to Step 4 - General Migration Settings Advanced Settings System and adjust the Simultaneous Migrations per Configuration Limit option.