Add this to your WordPress functions.php if you are using many custom image sizes (add_image_size(...)) to prevent the creation of all the sizes upon image upload. This will only generate a custom size the first time it is requested.

WordPress On-Demand Custom Image Sizes

/* ======================================================================================
    //! -- On Demand Image Sizing --

    Experimental code for generating custom image sizes on demand.
      http://wordpress.stackexchange.com/a/124790

    From the author...
      "When an image is then requested in a particular size,
       which is not yet generated, it will be created only that once."
   ====================================================================================== */
add_filter('image_downsize', 'ml_media_downsize', 10, 3);
  function ml_media_downsize($out, $id, $size) {
    $dims = null;
    $crop = false;

    if( is_array($size) ){
      // don't handle these for now. but if you want on-the-fly sizes, comment out
      // the return statement
      return false;

      $dims = $size;
      // create custom image size name consisting of widthXheight
      $size = sprintf("%dx%d", $size[0],$size[1]);
    }

    // If image size exists let WP serve it like normally
    $imagedata = wp_get_attachment_metadata($id);
    if (is_array($imagedata) && isset($imagedata['sizes'][$size]))
        return false;

    // Check that the requested size exists, or abort
    if( $dims === null ){
      global $_wp_additional_image_sizes;
      if (!isset($_wp_additional_image_sizes[$size]))
          return false;
    }


    // Make the new thumb
    if( $dims ){

      // we got passed custom width/height as array...
      if (!$resized = image_make_intermediate_size(
          get_attached_file($id),
          $dims[0],
          $dims[1],
          $crop
      )){
        return false;
      }

    }else{

      if (!$resized = image_make_intermediate_size(
          get_attached_file($id),
          $_wp_additional_image_sizes[$size]['width'],
          $_wp_additional_image_sizes[$size]['height'],
          $_wp_additional_image_sizes[$size]['crop']
      )){
        return false;
      }

    }

    // Save image meta, or WP can't see that the thumb exists now
    $imagedata['sizes'][$size] = $resized;
    wp_update_attachment_metadata($id, $imagedata);

    // Return the array for displaying the resized image
    $att_url = wp_get_attachment_url($id);
    return array(dirname($att_url) . '/' . $resized['file'], $resized['width'], $resized['height'], true);
  }


add_filter('intermediate_image_sizes_advanced', 'ml_media_prevent_resize_on_upload');
  function ml_media_prevent_resize_on_upload($sizes) {
      // Removing these defaults might cause problems, so we don't, though you could
      // probably set them to all be the same dimensions and rely on custom image sizes
      // for everything else
      return array(
          'thumbnail' => $sizes['thumbnail'],
          'medium'    => $sizes['medium'],
          'large'     => $sizes['large']
      );
  }