media_handle_upload() with 4 images under 4Mb total crashes server / timeout err

I’m reallu struggling with 500 Timeout Errors when uploading 4 images of less than 800kb each… I’ve tried placing ini_set('max_execution_time', 0); at the top of my code right after session_start(). I also tried setting php_value max_execution_time 300 in .htaccess and increasing the value in cPanel. I’m at a loss…

I wrote a function that performs all the checks and then uses media_handle_upload() to upload the files. The function is called 4 times in the code later with lines like this one:

if ( isset($_FILES["dlkimage1"]) && !empty($_FILES["dlkimage1"]["tmp_name"]) )
          fileUpload($_FILES["dlkimage1"],'dlkimage1');

I have also tried checking whether the $_SESSION ID of the image ID was set (which I set at the end of the upload function when a file was correctly uploaded) on the following upload, like this:

if ( isset($_FILES["dlkimage2"]) && !empty($_FILES["dlkimage2"]["tmp_name"]) && 
     isset($_SESSION['imagesuploaded']['dlkimage1_wp_id']) )
          fileUpload($_FILES["dlkimage2"],'dlkimage2');

But even with that extra check it gets stuck. When I look at the WP uploads folder I see the first image gets uploaded and nothing after that…

Here is the function:

function fileUpload($file,$which_image) {
        global $images_upload_folder;
        global $max_file_size;
        define ('SITE_ROOT', realpath(dirname(__FILE__)));
        $target_dir = $images_upload_folder;
        if ( isset($file) && !empty($file["tmp_name"]) ) {
            $filename = preg_replace('/\s+/','_', $file["name"]);   // remove white spaces from file name and replace with '_'
            $target_file = $target_dir . basename(cleanString(hyphenize($_SESSION['artist']).'_'.hyphenize($filename))); // where our file will live and what it will be called
            $imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));   // image extension

            echo '<small>';

            // Check if image file is a actual image or fake image
            $check = getimagesize($file["tmp_name"]);
            if( $check !== false ) {
                // echo "File is an image - " . $check["mime"] . ".";
                $uploadOk = 1;

                // Check if file already exists
                if ( file_exists($target_file) ) {
                  echo '<br /><span class="error">A file with the same name already exists.</span>';
                  $uploadOk = 0;
                }

                // Check file size
                if ( $file["size"] > $max_file_size ) {
                  echo '<br /><span class="error">This file is too large. Please keep files under '.( $max_file_size / 1000000 ).'Mb.</span>';
                  $uploadOk = 0;
                }

                // Allow only certain file formats
                if ( !empty($file) && $imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg" && $imageFileType != "gif" ) {
                  echo '<br /><span class="error">Only JPG, JPEG, PNG & GIF files are allowed.</span>';
                  $uploadOk = 0;
                }

                // Check if $uploadOk is set to 1 and, if so, proceed!!!
                if ( $uploadOk == 1 ) {
                    if ( !empty($file) ) {
                        $wp_image_array = array(
                            'post_author' => $_SESSION['writer_WP_id'],
                            'post_title' => $_SESSION['artist'],
                            'post_status' => 'inherit',
                            'post_name' => hyphenize($_SESSION['artist']).'_'.hyphenize($_SESSION['imagesuploaded'][$which_image]['name']).'_'.$which_image
                        );

                            $image_id = media_handle_upload(
                                        $which_image,       // file to upload
                                        $post_id,           // ID of image post
                                        $wp_image_array );  // image-specific data
                            if ( is_wp_error($image_id) )
                                echo '<br /><span class="error">There was an error uploading your file. - ' . $image_id->get_error_message() .'</span>';
                            else
                                echo '<br /><span class="success">Thank you for uploading your <b>'.str_replace('dlk','',$which_image).'</b>.</span>';

                    }
                } else {
                    echo '<br /><span class="error">Sorry, your file could not be uploaded.</span>';
                    $uploadOk = 0;
                }

          } else {
            echo '<br /><span class="error">This file is not an image.</span>';
            $uploadOk = 0;
          }

            echo '</small>';

        }
    if ( $uploadOk == 1 ) {
        // adds WHICH and WHAT image it is in the images uploaded array
        $_SESSION['imagesuploaded'][$which_image] = $file;  
        // add the WP image id into the session         
        $_SESSION['imagesuploaded'][$which_image.'_wp_id'] = $image_id;
        return $_SESSION['imagesuploaded'];
        unset($target_dir);
    }
}

Where am I failing?