I have noted a very significant performance difference when the iSCSI target LUN on the Synology is "file level" on a btrfs volume (using a "simple" LUN, not an "advanced" lun). I'm talking about >100% difference in performance compared to block level LUNs.
With a Raid6 over 12 6TB WD Red Pros, I get > 1GB/s, so the bottleneck is the 10gbps link in my case.
Also, for measuring throughput, I recommend Atto Disk Bench. Make sure you use a large test file (4 - 8GB, so that you can rule out cache). The interessting block sizes are >64k.
I get my 1GB/s at a block size of about 1MB/s (which is not uncommon for the backup job).