By chance are you using the "Primary" interface as well? I've seen an issue where when you temporarily shut down the Primary interface from the switch, the transfer stops due to an incorrect route configurations. Meaning you might be routing out the incorrect interface depending on your direction.
However, I had a very similar issue and was able to get a stable speed of 400MB/s checking the following. I should note that I still feel like there's still a bit of room for improvement.
I'd also check:
- MTU sizes for the entire path (Hosts, Switches, AltaVault, Etc)
- Jumbo frames for the Windows host if you are using 9000
- Network interfaces for the entire path (Hosts, Switches, AltaVault, Etc.)
- That SMB signing is not set for the Windows hosts which it sounds like you've already checked
- Check any group policy that might have an affect.
- Turn of anything that might proxy the traffice to verify the speed. I.E. (McAfee DLP, etc)
I'd also consider a slightly larger file to transfer do to the burst.
I'd also check this article
I'm not sure if this will help but I hope it does.