@Gankra depends...

xz and lzma2 are pretty good when it comes to compression ratio, and with modern hardware they are sufficiently fast.

• But there's a reason a lot of stuff, like online games, use bz2 instead, since that allows speeds in excess of 600 Mbit/s even on low-end systems with minimal load, and in my experience allows compression of backups like #PostgeSQL's cluster_backup at basically I/O speeds with <10% CPU load even at very old servers...

So there's that...

AFAICT xz is basically at the limit of lossless data compression - anything more efficient I know are extremely lossy narrowband codecs like #Codec2 @450 bit/s TWELP @ 300 bit/s and #STANAG4591 aka. MELPe that sound like one has their ears filled with water and the person talking has a gag ball in their mouth and screams.