Team? I'm just some guy throwing suggestion your way so maybe you can find out the cause quicker!
Have you tried transferring files wirelessly? Just curious to see if you're experiencing the problem even then.
You haven't tested with Jumbo Frames enabled, so that's something else that could be tested.
If both of these suggestions don't pan out for you, I'm fresh out of ideas and will therefore wish you luck in finding a solution with or without Synology's help.