GD image_png медленно

Я изменяю размеры нескольких изображений, и иногда image_png занимает 13 секунд, чтобы вернуть изображения, в то время как большинство возвращается через 0-1 секунд. Размер изображения с соотношением сторон 320/480 изменяется на 320×480 за 0 секунд, а до 750×1334 и 1242×2280 за 10-13 секунд.

Замедление происходит не по размеру изображения (0 секунд), а только с помощью функции imagepng.

Код

function generate_image_thumbnail($source_image_path, $thumbnail_image_path,$new_width,$new_height,$opts = Array())
{
$remove_transparency = isset($opts['remove_transparency']) ? $opts['remove_transparency']:false;
list($source_image_width, $source_image_height, $source_image_type) = getimagesize($source_image_path);
switch ($source_image_type) {
case IMAGETYPE_GIF:
$source_gd_image = imagecreatefromgif($source_image_path);
break;
case IMAGETYPE_JPEG:
$source_gd_image = imagecreatefromjpeg($source_image_path);
break;
case IMAGETYPE_PNG:
$source_gd_image = imagecreatefrompng($source_image_path);
break;
}
if ($source_gd_image === false) {
echo "Image Failure: $source_image_path\n";
echo "Image type: ".$source_image_type."\n";
exit();
}
$source_aspect_ratio = $source_image_width / $source_image_height;
$thumbnail_aspect_ratio = $new_width / $new_height;
if ($source_image_width <= $new_width && $source_image_height <= $new_height) {
$thumbnail_image_width = $source_image_width;
$thumbnail_image_height = $source_image_height;
} elseif ($thumbnail_aspect_ratio > $source_aspect_ratio) {
$thumbnail_image_width = (int) ($new_height * $source_aspect_ratio);
$thumbnail_image_height = $new_height;
} else {
$thumbnail_image_width = $new_width;
$thumbnail_image_height = (int) ($new_width / $source_aspect_ratio);
}
$thumbnail_image_height = $new_height;
$thumbnail_image_width = $new_width;
$thumbnail_gd_image = imagecreatetruecolor($thumbnail_image_width, $thumbnail_image_height);
$time = time();
imagecopyresampled($thumbnail_gd_image, $source_gd_image, 0, 0, 0, 0, $thumbnail_image_width, $thumbnail_image_height, $source_image_width, $source_image_height);
$time = time() - $time;
echo "Resample Time: $time\n";

$time = time();
$result = imagepng($thumbnail_gd_image, $thumbnail_image_path, 9);
$time = time() - $time;
echo "Image Time: $time\n";

imagedestroy($source_gd_image);
imagedestroy($thumbnail_gd_image);

if (!file_exists($thumbnail_image_path))
{
var_dump($thumbnail_gd_image);
}
return true;
}