The lz4 developers are the zstd developers :) In this case I suspect the IO reductions on the output are what matter -- while lz4 is faster, it also produces more output (of course) which you then have to write to disk. This can make the wall time take longer... basically you become IO bound on output, be it network or disk.

zstd also can tune itself and adjust ratios to saturate output bandwidth, which is pretty cool (--adapt).