diff --git a/phpBB/includes/auth.php b/phpBB/includes/auth.php
index 5b197e93f3..22aca5faf9 100644
--- a/phpBB/includes/auth.php
+++ b/phpBB/includes/auth.php
@@ -109,6 +109,7 @@ class auth
 	*/
 	function _fill_acl($user_permissions)
 	{
+		$seq_cache = array();
 		$this->acl = array();
 		$user_permissions = explode("\n", $user_permissions);
 
@@ -125,8 +126,17 @@ class auth
 
 				while ($subseq = substr($seq, $i, 6))
 				{
+					if (isset($seq_cache[$subseq]))
+					{
+						$converted = $seq_cache[$subseq];
+					}
+					else
+					{
+						$converted = $seq_cache[$subseq] = str_pad(base_convert($subseq, 36, 2), 31, 0, STR_PAD_LEFT);
+					}
+
 					// We put the original bitstring into the acl array
-					$this->acl[$f] .= str_pad(base_convert($subseq, 36, 2), 31, 0, STR_PAD_LEFT);
+					$this->acl[$f] .= $converted;
 					$i += 6;
 				}
 			}