When you import from Box.com, you might encounter a considerable slowdown in your migration. The lowered performance is often caused by throttling from the Box.com servers.
Throttling is normal, and you should not worry if you experience it. Throttling is an essential security mechanism that ensures the service offered by your server remains healthy and responsive. It is referred to as "throttling" because it limits the workload coming through the server. Without this, the server could become unresponsive or even crash completely---causing service interruptions for all users.
If you encounter significant performance issues with your Box.com migration, try setting the performance level to Minimal in the migration tool's settings.
Note: Anything above the Minimal performance setting will not resolve the slowdown caused by throttling. It is better to run multiple migration instances in Minimal performance mode than using Low impact in one migration instance if you are affected by Box.com throttling.