head.raku

multi fac($n where $n <= 1) { 1 }
multi fac($n) { $n * fac($n - 1) }
for ^100_000 {
    fac(10)
}
say now - INIT now;

new-disp.raku

use nqp;
multi fac($n where $n <= 1) { 1 }
multi fac($n) { $n * nqp::dispatch('raku-call', &fac, $n - 1) }
for ^100_000 {
    nqp::dispatch('raku-call', &fac, 10);
}
say now - INIT now;

results

jnthn@homedev:~/dev/rakudo$ ./rakudo-m where-head.raku 
4.859758291
jnthn@homedev:~/dev/rakudo$ RAKUDO_NEW_DISP=1 ./rakudo-m where-new-disp.raku 
1.337807436