This message was posted on behalf of Intel Corporation
Thank you for posting in Wired Ethernet Community.
Lowering the value or turning off interrupt moderation is best for handling many small packets so the driver is more responsive to incoming and outgoing packets.
You may also look into disabling other I219-V features such as offloading, energy efficient ethernet and flow control to check if it help in your scenario.
On the reason why other systems has better network performance, you can compare the features of the network adapter and GigE dongle vs I219-V for further isolation.
I'd suggest contacting ASUS support for similar issue reported and solution as our I219-V is an Ethernet Controller, further customization might have done to better fit into their system.
Thanks for the reply and checking back. I never received email notification so didn't realize you had replied.
I haven't resolved this. Still baffled by why much slower machines report better download speeds.
I'm starting to suspect there's something in the moderation behavior that has a sort of "beat frequency" with the sites used to test download speeds. With some sites I get better results, but the main comcast test site does very poorly with this particular i219-V.
I'm just going to live with it for now.
BTW, ASUS support was not helpful