Initially, we do have 7 nodes and later added 5 more nodes. Each node of size 36TB of 10 disks and 256GB RAM. Befrore adding new nodes, to 74GB of data it is taking below 10 mins, After adding new nodes, it took 2 hrs to copy data into hdfs.
can you check if rebalancing as kicked off or not
you should be able to grep this on namenode "org.apache.hadoop.hdfs.server.balancer.Balancer"..if the balancer has kicked off and is in progress, the process will be visible
Are you measuring the time taken by a single writer to transfer 74GB into HDFS? 12x degradation sounds like something is wrong.
Check the NameNode web UI to see if HDFS looks healthy and all DataNodes are heart-beating regularly. Also check your client,NameNode and DataNode logs for exceptions.
Are you running balancer? If so check the balancer settings to ensure they are not too aggressive.
I am able to see slow block reciever rate on datanode logs.
Another question that comes to mind is "are these new nodes in the same rack?". May be before you had all nodes in one rack but now you have cross rack trafiic also. Hadoop is rack aware so when you are adding new data, blocks are copied over to nodes in another rack also to ensure availability in case of rack failure.
Now if new nodes are in a different rack and balancer is also running, then that would explain why you are seeing slow performance.