Thank you for gathering those packet traces for me.
According to the packet traces, it appears the application
is doing exclusively stable writes (File_Sync) - server must
commits the data written and all the filesystem metadata to
stable storage before returning results. Basically, it means
every write has to be written to the disks before the next
write, which is a costly operation.
By default nfs does unstable writes, however, you can use
mount option "sync" to change it to stable writes. I have
reviewed your mount options and "sync" option is not used.
The other possibility is the application itself is doing the
stable writes.