When creating a D8 private file system e.g. /sites/default/private -> CiviCRM overrides location for templates_c specified in civicrm.settings.php file

Is this intentional? And is it necessary? I first saw this on my Pantheon test install and am now seeing it on one of Alan's hosted sites as well. Storing templates_c in the private file system [even though the civicrm.settings.php file still has the non-public files location] results in a slow page load.

This only happens if you put your private files dir in /sites/default/

On Alan's hosting we can move the private/files to the same level as web/

[$settings['file_private_path'] = $app_root . '/../private/files';

• so CiviCRM keeps using /sites/default/files for templates_c - but on Pantheon [and likely other hosting] you may not have access to create a dir at that level.

End result is that it creates the impression that CiviCRM on D8 is very slow.

@DaveD @seamuslee @jackrabbithanna @bgm - any ideas as to why/history/intentional?