When the load on the storage system increases, the I/O performance (response) to the host server might decrease. When you migrate volumes by using Volume Migration while the load on the storage system is high, the I/O performance (response) to the host server might decrease in higher possibility.
If the load on the storage system is high due to the I/O from hosts, do not migrate volumes by using Volume Migration. If you migrate volumes, the migration might fail and not be completed. If volume migration fails, temporarily decrease the load on the storage system, and then retry volume migration.