The Performance section of the spec (https://github.com/BLAKE3-team/BLAKE3-specs/blob/master/blake3.pdf https://github.com/BLAKE3-team/BLAKE3-specs/blob/master/

The Performance section of the spec (https://github.com/BLAKE3-team/BLAKE3-specs/blob/master/blake3.pdf https://github.com/BLAKE3-team/BLAKE3-specs/blob/master/blak...) has a paragraph about short input performance. 64 bytes happens to be the BLAKE3 block size, and performance at that length or shorter is best in class. Look at the left edge of Figure 3 (https://i.imgur.com/smGHAKA.png https://i.imgur.com/smGHAKA.png).