SQL restore time takes longer for larger database in general but the duration (clone-split) varies depending upon the size of the DB (LUN) and also other factors such as : How busy is your Cluster Node during the Restore process . In terms of available resources on Cluster Node, Ontap will give preference to front-end read/writes compared to restores.
1) Could you let us know the ONTAP version ?
2) Is the LUN on Spinning disk/Hybrid/SSD ?
3) Is the Volume containing the LUN "compressed" ?
4) During this restore - Do you have any replication or any other restores running in background ?
5) Could you share the resource utilization data for the Cluster Node/Volume (Latency/Mbs) during the restore process from OCUM/AIQM ?