Performance Considerations

Follow these simple steps and your site performance when using schema_metatag module should be bearable:

1. Enable only the schema_metatag submodules you need. The modules you enable would normally map to the top-level content types on your site.
2. For the most part you should only need a couple Schema.org groups on any given content type.
3. Try to be as specific as possible when you mark things up, and generally only  mark up the primary content on the page. This is not only a performance strategy, but also good SEO.
4. Add $conf['metatag_extended_permissions'] = TRUE; to settings.php, and give permissions for only the metatags your editors need. Be warned, if you have a lot of meta tags available in your site then your permissions page will grow.

If used wrongly, the schema_metatag module can make editing pages on your site a painfully slow experience. The problem is that the metatag module creates a gigantic form element with all possible values and displays that on each metatag field.

Some day this issue in metatag will be resolved, and performance will get even better. The D8 demo site has every module enabled and is still easy to use because it takes advantage of the metatag settings. (admin/config/search/metatag/settings).