Pls check this blog which I explained about the NFS3 slowness and we were constantly testing every update and working with VMware.
Recently we have upgraded the vCenter to update 3f and tested the NFS4 with NFS3 and we are happy with the results.
On addition to that NFS4 datastore disconnect during the Netapp upgrade, pls check this blog for more details and in the upgrade notes it is mentioned that issue has been fixed while failover.

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