Dynamic snitching 

Monitors the performance of reads from the various replicas and chooses the best replica based on this history.

By default, all snitches also use a dynamic snitch layer that monitors read latency and, when possible, routes requests away from poorly-performing nodes. The dynamic snitch is enabled by default and is recommended for use in most deployments. For information on how this works, see Dynamic snitching in Cassandra: past, present, and future. Configure dynamic snitch thresholds for each node in the cassandra.yaml configuration file.

For more information, see the properties listed under Failure detection and recovery.

The location of the cassandra.yaml file depends on the type of installation:
DataStax Enterprise 5.0 Installer-Services and package installations /etc/dse/cassandra/cassandra.yaml
DataStax Enterprise 5.0 Installer-No Services and tarball installations install_location/resources/cassandra/conf/cassandra.yaml
Cassandra package installations /etc/cassandra/cassandra.yaml
Cassandra tarball installations install_location/resources/cassandra/conf/cassandra.yaml