[KevinH]

Well those differences seem pretty significant. Perhaps you are right and there is no optimized sha256 on macOS arm64.

I will go ahead and add a user preference to enable / disable it.

But looking at what file_info is stored in zip and is freely available:

typedef struct unz_file_info_s
{
    uLong version;              /* version made by                 2 bytes */
    uLong version_needed;       /* version needed to extract       2 bytes */
    uLong flag;                 /* general purpose bit flag        2 bytes */
    uLong compression_method;   /* compression method              2 bytes */
    uLong dosDate;              /* last mod file date in Dos fmt   4 bytes */
    uLong crc;                  /* crc-32                          4 bytes */
    uLong compressed_size;      /* compressed size                 4 bytes */
    uLong uncompressed_size;    /* uncompressed size               4 bytes */
    uLong size_filename;        /* filename length                 2 bytes */
    uLong size_file_extra;      /* extra field length              2 bytes */
    uLong size_file_comment;    /* file comment length             2 bytes */

    uLong disk_num_start;       /* disk number start               2 bytes */
    uLong internal_fa;          /* internal file attributes        2 bytes */
    uLong external_fa;          /* external file attributes        4 bytes */

    tm_unz tmu_date;
} unz_file_info;

I could create a key from the pre-calculated crc32 value converted to a hex string, and append the basename (not the full path) of the file, and the file size in bytes again converted to a string.

Then I could add and use a crc32 checksum calculator only when saving the epub.

That should pretty much make speed a non-issue. Might be worth a shot.