Without having any log data I'm making some assumptions, so take it as hints to verify, but here goes! Your hardware config is an entry level system with only 7 data disks of 2TB SATA. So not a lot of horsepower in the controller and with few slow disks. Becauase your ODX performance is fine I assume that your LUNs are located in the same volume. If this is true, then with ODX enabled the system is internally using cloning which only has to update metadata and not actually read or write any data blocks from the disks. With ODX disabled then the system has to read the data blocks from disks, send it to the host, and then the host had to write them back all the way to disk. So without ODX there is much more disk IOPs and CPU needed and my guess is you are hitting the limits of your config.
As a simple test you could check sysstat command on the node while running the test:
Watch the columns I've highlighted, and the CPU column I forgot to highlight, to see if one of these is at limit during your test. If from the above you are still stuck I would open a support case and provide a perfstat so they can better explain what is going on.
Hope this helps!
Cheers, Chris Madden
Storage Architect, NetApp EMEA (and author of Harvest)