NFS has a ``sync'' option that specifies that the server should put data on the disk before replying that an operation is complete. This means that the actual I/O cost on the server side cannot be hidden with caching, etc. when this option is selected.
In the ``async'' mode the server can get the data into a buffer (and perhaps put it in the write queue; this depends on the implementation) and reply right away. Obviously if the server were to go down after the reply was sent but before the data was written, the system would be in a strange state, which is why so many articles suggest the "sync" option.
Some systems default to ``sync'', while others default to ``async'', and the default can change from version to version of the NFS software. If you find that access to an NFS volume through MPI-IO is particularly slow, this is one thing to check out.