Spring ThreadPoolTaskExecutor response time increases a lot

Question

I have configured Spring ThreadPoolTaskExecutor, trying to have in mind the average load of 1,1k rpm.

  @Qualifier("executor")
  public Executor asyncExecutor() {

    this.threadPoolTaskExecutor.setCorePoolSize(800);
    this.threadPoolTaskExecutor.setMaxPoolSize(900);
    this.threadPoolTaskExecutor.initialize();
    ..
  }

  //sender method
  @Async("executor")
  @Override
  public void sendObject (ObjectFormatter object) {

    executor.execute(() -> {
      try {
        items.forEach(item -> item.send(object));
    });

However, after a period of time the POST request for which the async configurations are made suddenly increases up to 5 hours (from the average ms period). More so, usually after those spikes the app stops sending the POST requests.

Response time picture:

What could be the cause of it? I have tried to play around with the core and max pool sizes, but eventually sooner or later the behaviors happens again.