Sorry, I was thinking about another thread when I wrote that reply. It should be UCARB for the coherence operation, which you probably want to make sure is elevated to ensure the coherence operations get done efficiently. But it could also be blocking from other activities w/ the MFENCE, but as you mentioned, that doesn't sound like it's highly active, but would be the MDMAARB.
I'd focus on change and testing the UCARBU first.
Best Regards,
Chad