Slow netapp repo?

[hnhn99]

I have a installation where backup jobs have gradually started taking longer and longer to complete over time.
On their physical proxy server, they are using iSCSI disks with LUNs from a NetApp FAS2720 (SATA drives), formatted with ReFS using a 64KB block size.
There are three LUNs/disks in total, and they are part of a Scale-Out Backup Repository.

Backups are especially slow between Monday and Tuesday for some reason, and I haven’t been able to figure out why.
Sure, SATA drives aren’t exactly fast, but if they were the root cause, I’d expect things to be slow across all days — not just early in the week.

There are no health checks scheduled, and synthetic fulls are created on Saturdays, but they finish well before Tuesday.

Would it be a good idea to disable deduplication on the NetApp volume and let Veeam handle it instead?

Also, would you recommend ReFS or NTFS for this type of setup?

How would you approach this situation?

[mjr.epicfail]

ReFS is the way to go for this because of the added feature of blockcloning that will greatly benefit you synthetic operations.
You could check the eventlogs or check performance of the Netapp via liveoptics?