I really should have read the documentation! That feature looks awesome, but in a quick test it could only use about 50% of the available output bandwidth. My upload speed is 50 Mbps, but zstd could only send about 25 Mbps.
Similarly, on a local speed test (SSD -> SSD), using a fixed compression level was much faster than --adapt.
Similarly, on a local speed test (SSD -> SSD), using a fixed compression level was much faster than --adapt.