Processing speed of large batch of agenda.now() jobs drops dramatically as time goes by

agenda-debug.js

const Agenda = require(`agenda`);

const mongoose = require(`mongoose`);
mongoose.connect(process.env.MONGO);

const agenda = new Agenda({
  maxConcurrency: 1,
  db: {
    address: process.env.MONGO,
  },
});

let lastJobCount;


// agenda.processEvery(`30 seconds`);
agenda.defaultLockLifetime(5e3); // 5 second lock

agenda.define(`TEST JOB`, { concurrency: 1 }, function(job, done) {
  setTimeout(()=>{
    done();
  }, 500)
});

agenda.on(`ready`, () => {
  console.log(`${new Date()} Agenda is ready`);
  agenda.start();
  setTimeout(()=>{
    //
    for (let i = 0; i < 25000; i++) {
      agenda.now(`TEST JOB`, {id: i});
    }
  }, 10e3);

  setInterval(run, 60e3);

});


function run(){
  mongoose.connection.db
    .collection(`agendaJobs`)
    .count({lastFinishedAt: {$exists: false}})
    .then(c => {
      console.log(`${new Date()} ${c} unfinished jobs in DB.`);
      if (lastJobCount && c) {
        let perM = lastJobCount - c;
        console.log(`${new Date()} processed job at ${perM} / min, ETA finish in ${c / perM} minutes`);
      } else if (c === 0) {
        console.log(`${new Date()} Jobs all done`);
      }
      lastJobCount = c;
    });
}

mongoose.connection.on(`connected`, () => {
  console.log(`${new Date()} Mongodb connected`);

});