yes, maybe I didn’t explain myself well.
LZ4, as @damonkohler mentioned, is already supported and works pretty well (I personally believe it should be the default one, rather then BZ4 that is indeed very slow).
My proposal is related to NOT compressed (i.e. “raw”) rosbags.
But since LZ4 is already very fast, maybe there is no need for improving the not compressed rosbag.