I just had a little look into these somewhat new zstd options --fast 1 to fast 5. fast 5 does seem almost equivalent to lz4 performance in compression ratio and compression 'speed' (or cpu load) except decompression 'speed' is still almost half of lz4, so at that extreme of zstds options it is squarely beaten by lz4. But in terms of compression ratio - which in most cases should be the more crucial characteristic for any compressor than cpu load - this setting seems to be well under zstd's knee of diminishing returns. --fast 1 has about 15% more cpu load for about 112% of compression performance than --fast5. And then zstds lightest non-fast setting (zstd-1) for 50% more load in compression and decompression, compresses 32% better than --fast5 (and lzo and lz4).

The decompression load is an eye catching feature of lz4, but its compression ratio is not just marginally compromised, it is a much less powerful compressor in that prime characteristic than zstds weakest non-fast setting. I can appreciate why Btrfs is in no hurry to include lz4 while it supports zstd:1 to 15. Although it would be interesting to test if lz4 could have a niche on fast storage combined with slow cpus.