Susanta, I don't have any answers but am experiencing the same issue.
NFS performance goes down the drain running on ESX 5.x with GAVG/rd and GAVG/wr breaking 1000ms.
We are currently migrating from ESX 4.0 to ESXi 5.1. In our Dev/Test environment installed vCenter 5.1 with a cluster of ESXi 5.1 servers. 3 x IBM HS22 8 cores, 96 GB ram. Have also migrated a few ESX 4.0 cluster to this vcenter.
Getting complaints from developers of sluggish servers on ESXi5.1. Checking counters, see disk latency through the roof > 1000 ms.
Create a test of a linux server, tar a local directory into the tmp folder. Run the test on the ESXi 5.1 host it takes over 12 mins to complete, with disk latency > 1000ms. Move the guest to an ESX 4.0 host using the same datastore job runs in 1.5 mins with normal disk latency. ESXi 5.1 host lightly loaded with no CPU or Memory contention.
Anyone else experiencing this or have any suggestions. Have an open ticket with VMware but still waiting a response.