NodeJS response takes a long time

I've made a simple request but sometimes, it takes a long time to get an answer. Chome DevTools shows me that TTFB (Time To First Byte) is often more than 10 seconds.

Here is my query. I'm using express and mysql packages.

  module.getSessionToken = function(request, response) {
       bddConnection.query(
           `SELECT id, firstname, lastname
            FROM users
            WHERE password = "${request.query.password}"
            AND mail = "${request.query.mail}"`,
           function(err, res) {
               if (err) throw err;
               response.send(res);
           }
       );
   };

This is the only request of my server with that issue. The others are working well.

via Valentin Duboscq