Holy Canoli You were right.

Checked the Synology’s network interface page and it was negotiating at 100 Mbps to my Orbi router. Your comment about 11.4 MB/s lining up with a 100 Mbps ceiling is exactly what got me to actually check the link speed instead of just assuming gigabit.

Bought a cheap unmanaged switch, plugged the NAS and server into it directly, came up at 1000 Mbps, and now I’m getting 107-115 MB/s on the same tests. Updated the post and credited you. Appreciate you pushing back on it.