Wordpress theme causing high server load - php

Question

I have a theme that I edited causing very high load on my server,
First i used this code to get only text from content
$response = get_the_content();
$content = $response;
$content = preg_replace("/(<)([img])(\w+)([^>]*>)/", "", $content);
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]>', $content);
Then i wanted to fetch all images inside my post content so i used "DOMDocument" Code:
$document = new DOMDocument();
libxml_use_internal_errors(true);
$document->loadHTML($response);
libxml_clear_errors();
$images = array();
$imgsq = $document->getElementsByTagName('img');
I have every post contains a static part than is in all photo pages so i used that code to get it
function findit($mytext,$starttag,$endtag) {
$posLeft = stripos($mytext,$starttag)+strlen($starttag);
$posRight = stripos($mytext,$endtag,$posLeft+1);
return substr($mytext,$posLeft,$posRight-$posLeft);
}
$project = #findit($content , '-projectinfostart-' , '-projectinfoend-');
$check = str_replace('ializer-buttons clearfix">', '', $project);
if($project != $check) $project = '';
$replace = array('-projectinfostart-' , '-projectinfoend-' , $project , '<p> </p>');
$content = str_replace( $replace, '', $content);
Then at last i wanted to get all photos in thumb size so i used that code:
foreach($imgsq as $key => $img) :
// Extract what we want
$image = array('src' => $img->getAttribute('src') );
if( ! $image['src'])
continue;
if($key == $page) :
echo '<center><img style="height: auto !important;max-height:450px;" class="responsiveMe" src=" ' . $image['src'] . '" /> ';
endif;
$srcs[$key] = array();
$srcs[$key]['src'] = wp_get_attachment_thumb_url(get_attachment_id_by_url($image['src']));
$srcs[$key]['full'] = $image['src'];
if(!empty($project)) $description = '<p>' . $project . '</p>';
else $description = '';
endforeach;
My server is 16 GB Ram and can't work with 1500 online users on single post page ! any ideas about what is causing this high load ?
Thanks.