Indeed, NFSoRDMA (NFS over RDMA) is no longer supported by MLNX_OFED since driver version 3.4.
If you are using the Inbox Driver provided by the OS (RHEL 7.4), we Mellanox support and/or assistance is on a best effort basis due to the Inbox driver support being handle through the OS vendor.
Why? The driver which comes with the RHEL OS is derived from the upstream kernel. The OS Vendor gets the driver from kernel.org and modifies the code to their own needs.
We do not control that code nor do we know what modification the vendor has done to that code.
The versions are also not the same between Mellanox OFED driver and INBOX driver.
That being said, a few recommendations and suggestions below:
Make sure HCA's FW are aligned and to the latest revision (Mellanox.com)
Make sure Switche(s) MLNX_OS are aligned and t the latest revision (Mellanox.com)
You can consult:
A) Performance Tuning for Mellanox Adapters
B) Red Hat Enterprise Linux Network Performance Tuning Guide
Performance Tuning Guide (Link)
D) How To Configure LACP on Mellanox Switches
E) Troubleshoot LAG/MLAG LACP-PDU Rate Issues
F) Perftest Package (RDMA)