In the mutation response, WriteResponse.java object is send back to the co-ordinator. This object has keyspace and key in it which is not required. It is not being used at the co-ordiantor.
This wastes IO specially in case of WAN links between DC. Also since response from each node in multi-DC deployments goes back to the co-ordinator in another DC makes it even worse.
It also becomes worse if the the keyspace and key are of large size and the data is small. In that case, a node which is not the co-ordinator and purely receiving mutations, the outbound n/w bandwidth could be half of incoming bandwidth.