Why is the write speed on NFS share slower than the write speed on local file system?

Issue

• Performing write operations on NFS shares is three times slower than the local file system.
• Why does write speed slow over NFS as compared to local file systems(ext3,ext4)
• Creating a file on local disk

$ dd if=/dev/zero of=/app/test.txt bs=1G count=1 oflag=direct
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 4.2535 seconds, 252 MB/s
$

• Creating a file on NFS mount point

$  dd if=/dev/zero of=/mnt/test.txt  bs=1G count=1 oflag=direct;date;rm /orabackup/t.txt
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 13.4193 seconds, 80.0 MB/s
$