Drupal's taxonomy system, to categorize and tag content, has one bad functiontaxonomy_get_tree(). It works fine on small sites with few tags. But on large sites with over tens of thousands of taxonomy terms, you should avoid it at all cost. The function namely stores all terms into the PHP RAM memory, quickly blocking further PHP execution.
Drupal core uses the function on several pages, such as the admin/content/node page and others. A solution is to use the Views Bulk Operations module (VBO) to replace that page with a custom view.
- Free PHP memory consumption by avoiding heavy taxonomy_get_tree() calls