we used an NFS area for some oracle external tables in Oracle RAC

performance was normal at Solaris OS level but terrible at RAC level.

below were our NFS mount options at OS level

  rw,bg,hard,nointr,rsize=32768,wsize=32768,proto=tcp,noac,forcedirectio,vers=4

Issue solved by disabling DNFS feature at Oracle external table level.