Mysqli + ajax request queue, implementation via response?

Question

AJAX

Good time of the day!
There was such a problem:
Through ajax, I set auto-update every 1 second from update requests, when there is 1 person on the resource, the server behaves quite normally, and when more than 3, a queue of requests is created that can be processed from 1 second (first request) to 1 minute (last request) ....
How can this be optimized or made sending a second request only when the answer came
PS I write in PHP

Answer 1

let id = 0;
const todoList = [];

function addNewToDo(data) {
  todoList.push({id: id++, index: todoList.lenght,data: data});
}

function remove(todo) {
delete todoList[todo.index];