Your doubly linked comment (adrian_b’s remark) makes the incorrect assertion that the only reason blake3 is faster than blake2 is parallelism; this is objectively false. Blake3 uses fewer rounds (7x2) of the Chacha primitive than blake2 (10x2).

I think tptacek has it right: this thing is used very infrequently (period 300 seconds) and blake2 was already in the kernel, so they just reused it. It’s adequately fast, even if blake3 would be marginally faster.

I do not think that he claimed that the reason is performance, but regardless, I gave other reasons besides performance, too, including "blake2 was already in the kernel, so they just reused it". I said: "BLAKE2 is already in the kernel (with tests and everything), while BLAKE3 is not (at all)".

Yes, we agree here.