Why is data transfer slow while transferring data in between NFS share from one data center to another?

Issue

• While transferring data internally in DATA CENTER A, getting a good speed

#  dd if=/dev/zero of=aaa.txt bs=1048576 count=1000
1048576000 bytes (1.0 GB) copied, 3.91387 seconds, 268 MB/s

• While transferring data internally in DATA CENTER B, speed is slow

# dd if=/dev/zero of=aaa.txt bs=1048576 count=1000
1048576000 bytes (1.0 GB) copied, 34.1162 seconds, 30.7 MB/s

• While transferring data in between the data centers speed is slow
  • From Data Center A to B

# dd if=/dev/zero of=aaa.txt bs=1048576 count=1000
1048576000 bytes (1.0 GB) copied, 16.4051 seconds, 63.9 MB/s

• From Data Center B to A

# dd if=/dev/zero of=aaa.txt bs=1048576 count=1000
1048576000 bytes (1.0 GB) copied, 18.1717 seconds, 57.7 MB/s