Embedding transparent png is too slow

Embedding transparent png is too slow

The function below embeds a watermark onto an image. In some servers it's
super slow while I never encounter the same issue in my server. What could
be wrong with it? I'm out of ideas.
function add_watermark( $source_file_path, $output_file_path,
$watermark_file_path, $quality = 80 ) {
list( $source_width, $source_height, $source_type ) = getimagesize(
$source_file_path );
if ( $source_type === NULL ) {
return false;
}
switch ( $source_type ) {
case IMAGETYPE_GIF:
$source_gd_image = imagecreatefromgif( $source_file_path );
break;
case IMAGETYPE_JPEG:
$source_gd_image = imagecreatefromjpeg( $source_file_path );
break;
case IMAGETYPE_PNG:
$source_gd_image = imagecreatefrompng( $source_file_path );
break;
default:
return false;
}
$overlay_gd_image = imagecreatefrompng( $watermark_file_path );
$overlay_width = imagesx( $overlay_gd_image );
$overlay_height = imagesy( $overlay_gd_image );
imagecopy(
$source_gd_image,
$overlay_gd_image,
$source_width - $overlay_width - 10,
$source_height - $overlay_height - 10,
0,
0,
$overlay_width,
$overlay_height
);
imagejpeg( $source_gd_image, $output_file_path, $quality );
imagedestroy( $source_gd_image );
imagedestroy( $overlay_gd_image );
}