edit.php and post-new.php load

This is on a new install with Twenty Fifteen theme and no activated plugins.
I used Really Simple CSV Importer to import ~4000 pages with title and one word content for each. This is to have my pages ready for use with Advanced Custom Fields (pages will have dozen text areas metaboxes, no relationship or advanced ACF features)
edit.php and post-new.php are very slow. I guess the number of pages are the factor. I wanted to see what effect deactivating ACF and all other plugins have. No noticeable difference, I turned off all screen options and reduced the ‘Number of items per page’ too, so now the edit.php only shows pages titles and post-new.php only the title and content boxes and Publish buttons.

I guess the question is the following, on both edit.php and post-new.php after all I have done above when I look at the page source a list of all my pages are loaded below this code:

<div id="pageparentdiv" class="postbox  hide-if-js" >
<div class="handlediv" title="Click to toggle"><br /></div><h3 class='hndle'><span>Page Attributes</span></h3>
<div class="inside">
<p><strong>Parent</strong></p>
<label class="screen-reader-text" for="parent_id">Parent</label>
<select name='parent_id' id='parent_id'>

Is it possible to not have that load every time? If so, using a filter or plugin?

TIA