When reading from media with high random-access latency (for instance traditional hard drives) going parallel could make things much slower, and it will reduce

When reading from media with high random-access latency (for instance traditional hard drives) going parallel could make things much slower, and it will reduce the compression achieved (if only by a small amount), and it will increase the amount of memory consumed during the process, so I wouldn't want it to be the default. Nor would I particularly want it to try be clever and detect the usefulness of going parallel as that could lead to unexpected inconsistency. I think there is a case for including it as a selectable option, as with --rsyncable, unless this adds extra dependencies (pthreads was mentioned in other comments).