Slowdown

euler14.pl6

# Longest Collatz sequence - Which starting number, under one million, produces the longest chain?
#real	2m18.424s
#user	2m17.600s
#sys	0m0.056s
# This is Rakudo version 2016.06-11-g2c61fda built on MoarVM version 2016.06
use v6;
my $maxChainLen = 0;
my $maxChainLenNum = 0;
my @cache is default(0);
for (1..1_000_000) -> $x {
  my $num = $x;
  my $chainLen = 0;
  while $num > 1 {
    if $num < 75_000 && (@cache[$num] != 0) {
      $chainLen += @cache[$num];
      last;
    }
    $chainLen++;
    $num = ($num % 2 == 0) ?? $num div 2 !! 3 * $num + 1;
  }
  $chainLen++ if $num == 1;
  @cache[$x] = $chainLen if $x < 75_000;
  if $chainLen > $maxChainLen {
    $maxChainLen = $chainLen;
    $maxChainLenNum = $x;
  }
}
say "Max chain length was $maxChainLen for number $maxChainLenNum";