Whilst using (niced-to-oblivion) pigz will give you a shorter compression time, I think it’s somewhat orthogonal to the decision to enable --rsyncable (modulo compressed size differences), because any benefit you get from using pigz is equally applicable to both --rsyncable and normal compression. Trading off more CPU time for better compression is an interesting idea, since you’ll still (presumably) get wall clock benefits. Still, though, since it’s a background process, conserving wall clock time isn’t as big a deal as it would be otherwise. I wonder if using gzip for background jobs, but pigz for interactive backups, would be a worthwhile complication…