Use gt operator instead of max for highest row

Using an operator is significantly faster than calling the max function.
As this method is called more than once per cell the difference adds up.

Closes #824

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - Change `libxml_disable_entity_loader()` as shortly as possible - [#819](https://github.com/PHPOffice/PhpSpreadsheet/pull/819)
 - Improved memory usage and performance when loading large spreadsheets - [#822](https://github.com/PHPOffice/PhpSpreadsheet/pull/822)
 - Improved performance when loading large spreadsheets - [#825](https://github.com/PHPOffice/PhpSpreadsheet/pull/825)
+- Improved performance when loading large spreadsheets - [#824](https://github.com/PHPOffice/PhpSpreadsheet/pull/824)
 
 ## [1.5.2] - 2018-11-25
 

src/PhpSpreadsheet/Worksheet/Worksheet.php

@@ -1268,7 +1268,9 @@ private function createNewCell($pCoordinate)
         if (Coordinate::columnIndexFromString($this->cachedHighestColumn) < Coordinate::columnIndexFromString($aCoordinates[0])) {
             $this->cachedHighestColumn = $aCoordinates[0];
         }
-        $this->cachedHighestRow = max($this->cachedHighestRow, $aCoordinates[1]);
+        if ($aCoordinates[1] > $this->cachedHighestRow) {
+            $this->cachedHighestRow = $aCoordinates[1];
+        }
 
         // Cell needs appropriate xfIndex from dimensions records
         //    but don't create dimension records if they don't already exist