We have a table which has lot of delete markers and we running Canary test on a regular interval sometimes tests are timing out because to reading first row would skip all these delete markers. Since purpose of Canary is to find health of the region, i think keeping raw=true would not defeat the purpose but provide good perf improvement.
Following are the example of one such scan where
without changing code it took 62.3 sec for onre region scan
2016-06-23 08:49:11,670 INFO [pool-2-thread-1] tool.Canary - read from region <tablename>.<region> column family 0 in 62338ms
whereas after setting raw=true, it reduced to 58ms
2016-06-23 08:45:20,259 INFO [pool-2-thread-1] tests.Canary - read from region <tablename>.<region> column family 0 in 58ms
Taking this over multiple tables , with multiple region would be a good performance gain.