If you want an equal interval, your loop is fine, you just need to fix the math for calculating the timeout:

const btn = document.getElementById('btn');
btn.addEventListener('click', function(){
  
    for (let index = 10; index > 0; index--) {
        setTimeout(() => {
            console.log(index)
        },  1000*(10-index));      
    }
})

<div class="app">
  <button id="btn">click</button>
</div>

That will count down from 10 to 1 once every second, just change the 1000 as needed if you want it to be faster or slower.

Note that the order of the loop doesn't actually matter here (aside from a few ms difference in execution time) since it's asynchronous - you could use a non-reversed loop and still get the same result to the human eye.

It's not that i-- is faster than i++. Actually, they're both equally fast.

What takes time in ascending loops is evaluating, for each i, the size of your array. In this loop:

for(var i = array.length; i--;)

You evaluate .length only once, when you declare i, whereas for this loop

for(var i = 1; i <= array.length; i++)

you evaluate .length each time you increment i, when you check if i <= array.length.

In most cases you shouldn't even worry about this kind of optimization.

var num = 10,
reverse = false;

if(!reverse) for( var i=0;i<num;i++) log(i);
else         while(num-- )      log(num);

   // to avoid duplication if the code gets long
function log( num ) { console.log( num ); }

EDIT:

As noted in the comments below, if i is not declared elsewhere and you do not intend for it to be global, then declare it with the other variables you declared.

And if you don't want to modify the value of num, then assign it to i first.

var num = 10,
reverse = false,
i;

if(!reverse) for(var i=0;i<num;i++) log(i);   // Count up
else         {var i=num; while(i--) log(i);}  // Count down

function log( num ) { console.log( num ); }

You can use it like this

for (var i = 10; i >= 0; i--) 

But I'm not sure why are you doing so because you are not using this variable inside your loop so I think it doesn't make any difference to the output as you loop going to run 10 times either way.

If I could guess then I think you want to print the stars in reverse order of quantity. That means you want to have maximum stars in first row then -1 in second and so on.. If so then you need to reverse the order of inner loop only like this

 function seethestars1() {
    var stars = document.getElementById("emptytext2");
    for (var i = 0; i <= 10; i++) {
        for (var j = 10; j > i; j--) {
            stars.value += "*";
        }                            
      stars.value += "\n";
    }
}

Js Fiddle Demo