High CPU load for Cassandra 2.1.8

We are seeing High CPU Load about 80% to 100% in Cassandra 2.1.8 when doing Data ingest, we did not had this issue in 2.0.x version of Cassandra
we tested this in different Cloud platforms and results are same.
CPU: Tested with M3 2xlarge AWS instances
Ingest rate: Injecting 1 million Inserts each insert is of 1000bytes
no other Operations are happening except inserts in Cassandra
let me know if more info is needed.