It will need proper investigation. You can raise a ticket with NetApp so that we can get to the bottom of this issue.
In the meantime, check out these kbs.
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Storage_Software/ONTAP_OS/Copying_data_to_CIFS_shares_is_slow
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Storage_Software/ONTAP_OS/How_to_troubleshoot_slow_access_to_files_via_CIFS%2F%2FSMB_protocol
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Storage_Software/ONTAP_OS/Copy_of_multiple_files_to_the_CIFS_share_path_is_very_slow
Please share the - NetApp Model and Ontap version.