My interpretation is that the setup is 10GbE UNAS Pro 8, 4x 1GbE Synology DS 1520, 1x 2.5GbE Windows host.

If that's the setup, the only way anything would run more than 1Gb is if the Synology is doing SMB-multichannel (which if they setup 4x1Gb LAG, then it's not SMB-MC). [edit] reading the article, they do have the Syno setup for SMB-MC and that's the only reason this wasn't stuck at ~120MB/s