Exec wp query in slow motion to avoid memory error?

It appears that you’re trying to get the latest 15 posts where the meta key is not equal to ‘None’.

Instead of loading all the posts into memory and then looping through to see if the meta value is not ‘None’, you could use the meta_query parameter.

$args = [
  'post_type'      => 'post',
  'orderby'        => 'date',
  'order'          => 'ASC',
  'posts_per_page' => '15',
  'meta_query'     => [
    [
      'key'     => 'slider_position',
      'value'   => [ 'None' ],
      'compare' => '!=',
    ],
  ],
];
$query = new WP_Query( $args );