NetApp recommends a NFS queue depth of 64 for all ESX hosts.  Here is a quick PowerCLI script to set it on all hosts.  This change requires a reboot of the ESX host to take effect.

Connect-VIServer vcenter.domain.tld
$esxHosts = Get-VMHost | Sort Name
foreach($esx in $esxHosts){
    if((Get-VMHostAdvancedConfiguration -VMHost $esx -Name NFS.MaxQueueDepth).Values -ne "64"){
        Set-VMHostAdvancedConfiguration -VMHost $esx -Name NFS.MaxQueueDepth -Value 64 -Confirm:$false
    }
}