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While we're checking this internally, please also contact VMware support or VMware communities to report your issue.
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Particular issue is resolved and was happening on ESXi 6.5 (Beta Version).
Also the issue was irrespective of Intel Adapters as we faced the same issue on a different Non-Intel setup.
The moment we upgrade ESXi 6.5(Beta) ->ESXi 6.5(Released) version we started seeing around 32 Gbps of throughput with MTU as 9000 bytes and with further performance tuning it goes upto 37 Gbps
I thought its worth sharing the information so as to save time for the users facing similar issue.