Can Javascript control animation loop & frame rate?

I followed an article about getting the best out of delta time in animation loops.
It’s the most advanced way to handle loop renders and updates I’ve ever read.
https://isaacsukin.com/news/2015/01/detailed-explanation-javascript-game-loops-and-timing

I implemented it within a class and it works.
However:

I would like to understand why I have sudden FPS drops when adjusting values:

1. I am on an iMac Pro which has a refresh rate of 60hz. But, when I set "maxFPS" to 60 (or any value below 70), my frame rate drops in 5 seconds from 60 FPS to 32fps. So, I need to increase "maxFPS" and I only get a steady 60fps when I set it above 70, to any value.

2. Whatever "maxFPS" I put, if I change the "if (timestamp > this.lastFpsUpdate + 1000)" statement to something slightly lower than 1000 (even 950), once, again, my frame rate drops dramatically around 32fps.

Messing with those 2 values lead to the EXACT same behavior.

FYI:

my canvas is 3840 x 2160 but, the 2 issues I mentioned here above are exactly the same at that resolution or at 960×540. The resolution doesn’t seem to affect the issues at all (I get the same frame rates and (lack) of performance no matter the resolution).

The fps drops seem so sudden for very specific parameter values that it feels like it’s a code issue rather than a performance issue. Indeed, I don’t see why a change from 1000ms to 950ms in the FPS display refresh rate would drop the FPS so dramatically all of a sudden. And I don’t see why setting the maxFPS to 60 is a problem on a 60hz screen while setting it to 70 or even 1500 is just fine. And these drops will consistently happen no matter if I actually render something or if my draw/update functions are completely empty.

Here is my loop class file:

class Loop{

    //--------------------------------------------------------------------

    constructor(maxFPS,displayFPS){

        this.fps              = 60;
        this.maxFPS           = maxFPS; //if lower than 70, the fps is dropping fast 
        this.displayFPS       = displayFPS;
        this.timestep         = 1000/this.fps;
        this.delta            = 0;
        this.lastFrameTimeMs  = 0;
        this.framesThisSecond = 0;
        this.lastFpsUpdate    = 0;

    }

    //--------------------------------------------------------------------

    mainLoop(timestamp) {
    
        if (timestamp < this.lastFrameTimeMs + (1000 / this.maxFPS)) {

            requestAnimationFrame(this.mainLoop.bind(this));
            return;

        }

        this.delta          += timestamp - this.lastFrameTimeMs;
        this.lastFrameTimeMs = timestamp;

        //any value lower than 1000ms is dropping the FPS here...
        if (timestamp > this.lastFpsUpdate + 1000) {

            this.fps              = 0.25 * this.framesThisSecond + 0.75 * this.fps;
            // same issue if I just put:
            //this.fps            = this.framesThisSecond;
            this.lastFpsUpdate    = timestamp;
            this.framesThisSecond = 0;

        }

        this.framesThisSecond++;

        var numUpdateSteps = 0;

        while (this.delta >= this.timestep) {
        
            // the update function of my working file
            update(this.timestep);

            this.delta -= this.timestep;

            if (++numUpdateSteps >= 240) {

                this.delta = 0;
                break;

            }

        }

        if(this.displayFPS == true){

            document.getElementById('frameRate').textContent = (Math.round(this.fps * 100) / 100).toFixed(2);

        }

        context.clearRect(0, 0, canvas.width, canvas.height);

        // the draw function of my working file
        draw(this.delta/this.timestep);

        requestAnimationFrame(this.mainLoop.bind(this));

    }

    //--------------------------------------------------------------------

    start(){

        // the setup function of my working file
        setup();
    
       requestAnimationFrame(this.mainLoop.bind(this));

    }

    //--------------------------------------------------------------------

}

Here is my working file:

//--------------------------------------------------------------------
// Initialize
//--------------------------------------------------------------------

const canvas = document.querySelector('canvas')
      canvas.width  = 3840; // tested: no impact on my issues 
      canvas.height = 2160; // tested: no impact on my issues 
const context       = canvas.getContext('2d');

const loop = new Loop(maxFPS=70, display=true);
      loop.start();

//--------------------------------------------------------------------
// Variables
//--------------------------------------------------------------------

var boxSize;
var boxPos;
var lastBoxPos; 
var boxVelocity;
var limit;

//--------------------------------------------------------------------
// Animation Test
//--------------------------------------------------------------------

function setup(){

    boxSize     = 200;
    boxPosX     = 10;
    boxPosY     = 10;
    lastBoxPosX = 10;
    boxVelocity = 1;
    limit       = canvas.width - boxSize;

}

//--------------------------------------------------------------------

function update(delta){
    
    boxLastPosX  = boxPosX;
    boxPosX     += boxVelocity * delta;
    if (boxPosX >= limit || boxPosX <= 0) {boxVelocity = -boxVelocity;}

}
//--------------------------------------------------------------------

function draw(interpolation){
    
    context.beginPath();
    context.fillStyle = "#3e4856";
    context.fillRect((boxLastPosX + (boxPosX - boxLastPosX) * interpolation), boxPosY, boxSize, boxSize);
  
}

//--------------------------------------------------------------------

Thanks for your input.