Disable Custom CSS is a WordPress plugin (download) that disables automatically disables frontend database query for Custom CSS and Customizer section for setting Custom CSS, without need to manually choose any setting or option.
It is a very lightweight, it has no settings, just activate it and it works immediately.
Disable Custom CSS is released under the terms of GNU General Public License. It is completely free so if you find it useful you can give something back to author. Your help could be done by giving donation, sending pull request on GitHub, sending bugs and suggestions, promoting plugin in blogs, forums, social networks, and other websites.