use strict;
use warnings;
use feature 'say';

use B 'hash';
use Crypt::xxHash 'xxhash3_64bits';
use Digest::xxH64 'xx64';
use Benchmark 'cmpthese';

use Inline C => <<'_C_';
U32 myhash(SV* sv) {
    STRLEN len;
    U32 hash = 0;
    const char *s = SvPVbyte(sv, len);
    PERL_HASH(hash, s, len);
    return hash;
}
_C_

srand 1234; 
my $s = pack 'C*', map rand 256, 1 .. 64; 

cmpthese -2, {
    hash   => sub { my $x = hash( $s )},
    myhash => sub { my $x = myhash( $s )},
    xxhash => sub { my $x = xxhash3_64bits( $s, 0 )},
    xx64   => sub { my $x = xx64( $s )},
};

__END__

             Rate   hash myhash xxhash   xx64
hash    1944302/s     --   -52%   -54%   -84%
myhash  4088577/s   110%     --    -3%   -66%
xxhash  4233986/s   118%     4%     --   -65%
xx64   11994386/s   517%   193%   183%     --

This is perl 5, version 32, subversion 1 (v5.32.1) built for MSWin32-x
+64-multi-thread
[download]

Try xxHash? The Digest::xxH64 is not on CPAN (but linked to from home i.e. officially 'endorsed'(?):)), Crypt::xxHash needs a fix to install in Windows, and Digest::xxHash (not in example above) is slower and therefore perhaps not of much interest in context of 'B::hash is too slow'.

As already mentioned, the Judy::HS provides both hashing and sparse storage already built-in under-the-hood. So maybe manually-done hashing is not what you need. I have 'played' (i.e. not in serious 'production') with Judy (but not with Judy::HS) to store and access huge sparse data, and, yes, speed is comparable to Perl hashes with significantly less RAM appetites.

Another option to consider: Math::GSL::SparseMatrix (and GSL being solid and renowned, etc.). As above, I 'played' with 64-bit-addressed sparse single-row (or was it single-column?) vector. Slower than Judy, yet installs without hassle in Windows, theoretically can address 128-bit sparse space (because of 2D) and can store data shorter than 64-bit integers i.e. needs even less RAM in that case.