In the spirit of
CASSANDRA-511, it would be useful to tracing on queries to see where latency is coming from: how long did row cache lookup take? key search in the index? merging the data from the sstables? etc.
The main difference vs setting debug logging is that debug logging is too big of a hammer; by turning on the flood of logging for everyone, you actually distort the information you're looking for. This would be something you could set per-query (or more likely per connection).
We don't need to be as sophisticated as the techniques discussed in the following papers but they are interesting reading: