If you have shared hosting, check with your hosting company. It’s possible there may be a server issue, or perhaps they can tell you whether you are approaching one of their internal resource limits.
Three internal WordPress features (WP-CRON, REST API, admin-ajax.php) can be extremely hard on a server if abused by plugins and themes. (A query monitor will be of no use if the issue is excessive usage of any of these three features.)
Many plugins and themes are hard on a server. Some examples are the Divi theme and the WooCommerce, Jetpack, Visual Composer, Gravity Forms, and Wordfence plugins.
If you have a VPS, you should look into optimizing the MySQL configuration. Due to the poor construction of its _postmeta and _usermeta tables, WordPress requires a different configuration that what is recommended by programs such as mysqltuner.
The following four suggestions apply to a VPS or dedicated server, or if you have shared hosting that allows the selection of PHP versions and extensions.
Make sure you are using PHP 7.0 or later. I suggest the latest version in the 7.1 branch, which is currently 7.1.22.
Make sure the “mysqlnd” and “mysqli” PHP extensions are installed, and the “mysql” extension is NOT installed.
Make sure to use an opcode cache such as Zend Opcache. This can easily be done on a VPS or dedicated server, and many shared hosting plans now offer access to Zend Opcache in their PHP configuration.
Make sure you limit the number of extensions that are installed. In particular, if you do not need them (and you probably don’t), remove the “fileinfo”, “intl”, and all extensions starting with “PDO”. These are extremely large extensions that consume memory on every page load that could otherwise be used for WordPress.
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Beyond this, I suggest you ask for assistance on serverfault.com or webhostingtalk.com, and if you have a VPS or dedicated server, consider hiring someone to optimize your server.
