How busy is the CPU?

No, the example of 4-20000 LEDs was just that: an example. I know I can change the brightness of the 4 LEDs I have via setting the duty cycle via setInterval(). No issues. CPU power is enough even for updating once in 5ms per LED.
I am quite confident thatI cannot do the same if I had 20000 LEDs. The question was: how do I know when the CPU won't be able to catch up anymore?
And the setBusyIndicator() does that.

BTW, I checked and this is the result:

// Glow 4 LEDs

var glow=function(led) {
  var intensity=0;
  var dir=1.0;
  var this_led=led;
  function intensity_step(n) {
    intensity+=dir*n;
    if (intensity>=1.0) {
      intensity=1.0;
      dir=-dir;
    }
    if (intensity<=0.0) {
      intensity=0;
      dir=-dir;
    }
    analogWrite(this_led, intensity);
  }
  return intensity_step;
};

var led1=glow(D13);
setInterval(function() {
  led1(0.01);
  }, 2);
  
var led2=glow(D14);
setInterval(function() {
  led2(0.01);
  }, 2);


var led3=glow(D12);
setInterval(function() {
  led3(0.01);
  }, 2);

var led4=glow(D15);
setInterval(function() {
  led4(0.01);
  }, 2);

// See how busy the CPU is

setBusyIndicator(D11);

Attached the wave form of D11 when changing only one LED. You can see this is repeating at 500Hz, and it's already using 25% of CPU.
4 of those keep the CPU close to 100%.

Please no one comment on the horrible code. I'm just learning JavaScript and I already figured out the code is sub-optimal in so many ways ^_^

PS: Forgot to mention: This is on the STM32F4 Discovery board.

1 Attachment