read_request_timeout_in_ms: The time that the coordinator waits for read operations to complete.
If you decrease it you will have more chances of facing timeout issues, whereas if you increase it the performance might get affected.
From Cassandra Document:
For instance, Cassandra’s timeout exception reflects the incomplete (but not failed) handling of a request. Timeouts occur when the coordinator node sends a request to a replica and does not receive a response within the configurable timeout window. Timeouts are not necessarily fatal—the coordinator will store the update and attempt to apply it later—but they can indicate network issues or even disks nearing capacity. The settings read_request_timeout_in_ms (defaults to 5,000 milliseconds), write_request_timeout_in_ms (defaults to 2,000 ms), and other timeout windows can be set in the cassandra.yaml configuration file.