Local (ext4, xfs, etc.) filesystem works much faster because it doesn't have to communicate back-and-forth for every single file.
The main downside is of course you can't share data with anyone that way. So using iSCSI may be fine if you need a local copy quickly, or if you can dismount the clone volume and let some other client mount it.
Years ago I used to do demos with Android source code and iSCSI and some users didn't even want to compare with their old approach - they were sure their workflow was faster 🙂