PrestaShop back office load a lot of files in some sections, but more important, have a API that calls addons.prestashop.com. If the Addons site is down or slow, your back office will be slow too. You can disable this API calls editing 2 files (in PS1.7).
Search in the file classes/Tools.php (PS 1.6 and 1.7) and in the file src/Adapter/Addons/AddonsDataProvider.php (only in PS 1.7)
and change the line:
protected static $is_addons_up = true;
to
protected static $is_addons_up = false;
This disable the API calls, so you dont get news from addons site.