diff --git a/Classes/PHPExcel/Calculation/LookupRef.php b/Classes/PHPExcel/Calculation/LookupRef.php
index af38ea36..d8605512 100644
--- a/Classes/PHPExcel/Calculation/LookupRef.php
+++ b/Classes/PHPExcel/Calculation/LookupRef.php
@@ -674,10 +674,10 @@ class PHPExcel_Calculation_LookupRef
     {
         reset($a);
         $firstColumn = key($a);
-        if (strtolower($a[$firstColumn]) == strtolower($b[$firstColumn])) {
+        if (($aLower = strtolower($a[$firstColumn])) == ($bLower = strtolower($b[$firstColumn]))) {
             return 0;
         }
-        return (strtolower($a[$firstColumn]) < strtolower($b[$firstColumn])) ? -1 : 1;
+        return ($aLower < $bLower) ? -1 : 1;
     }
 
 
diff --git a/changelog.txt b/changelog.txt
index d4afc0c0..2f455c2d 100644
--- a/changelog.txt
+++ b/changelog.txt
@@ -26,6 +26,7 @@
 Planned for 1.8.2
 - Bugfix:   (MBaker)                            - Fix to getCell() method when cell reference includes a worksheet reference
 - Bugfix:   (ncrypthic)       Work Item GH-570  - Ignore inlineStr type if formula element exists
+- General:  (umpirsky)        Work Item GH-548  - Optimize vlookup() sort
 
 
 2015-04-30 (v1.8.1):