Looks like, according to NetApp TR-4717, this is expected. A 1.5TB database restore operation generally takes about nine hours.
Page-20:
Restoring to an alternate host is a "copy-based" restore mechanism. Large databases might take more time to perform a restore operation because it’s a "streaming process".
For Dev/Test environment: To save restore (clone-split) time.
NetApp best practice is to leverage the cloning methodology to create a copy of the database on the same instance or alternate instance. At a later time, a user can perform a clone split during a peak period or maintenance window to isolate the clone copy from the Snapshot copy and avoid any dependency on either of them in future. To set the restoration timeout value so the large databases do clone split completely successfully, see the section titled, “Restoring a database by restoring to an alternate host option.”
Restoring a database by restoring to an alternate host option:
https://www.netapp.com/pdf.html?item=/media/12400-tr4714pdf.pdf
Why does flex volume clone split take a long time?
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Storage_Software/ONTAP_OS/Why_does_flex_volume_clone_split_take_a_long_time