config-dist.php has:

    'dbcollation' => 'utf8mb4_unicode_ci', // MySQL has partial and full UTF-8
                                // support. If you wish to use partial UTF-8
                                // (three bytes) then set this option to
                                // 'utf8_unicode_ci', otherwise this option
                                // can be removed for MySQL (by default it will
                                // use 'utf8mb4_unicode_ci'. This option should
                                // be removed for all other databases.

While removing this option does indeed use utf8mb4_unicode_ci (assuming Moodle was installed using the recommended settings), this requires an additional SELECT to read this setting during every connection to the database. This is confirmed by Eloy's comment in MDL-48228: "for sure having it in config.php is the quickest option, nothing to execute ever, the collation is already there, zero queries".