wp-content/mu-plugins and wp-content/plugins: using them correctly

Hi,

Sorry to post about this but I need a confirmation.

As far as I understand, all plugins placed into the wp-content/mu-plugins folder are executed automatically and sometimes on every page load (!)…. That means that putting all the plugins into this folder could be very resource intensive and dramatically slow down the entire website. Correct?

I’m asking this because I’ve seen several installation instructions stating that the plugin could be placed either in the mu-plugins folder or in the plugins folder, without any warning.

Example: https://wpmudev.com/project/subscribe-by-email/installation/

If you want Subscribe by email to be activated automatically site-wide for all blogs upload subscribe-by-email.php into wp-content/mu-plugins/

I think this is not a good reason to put this plugin into the mu-plugins folder. This plugin should be put into the normal plugins folder and activated sidewide by the super admin (network activate), shouldn’t it? What would happen if someone put all your 100+ plugins into the wp-content/mu-plugins/?

Well, I think it would be helpfull to know exactly what are the rare (?) WPMUDEV plugins that must be placed into this particular folder.