First, enable the detection of slow nodes. You can do this by setting the value of dfs.namenode.peer.stats.enabled property to true.
Let the cluster run for sometime.
To access the details, visit: http://<namenode_host>:50070/jmx
Here you will see the time taken by datanodes. After you have detected the datanodes that are slow, you have to check for the bottlenecks in data flow.