I was going through Eloquent JavaScript (again) and came across exercise "Chess Board" of Chapter 2. I had my one decent version of solution written back in the day when I was first time reading it, and another version of solution provided at the Elequent Javascript website. I'm one of the newbies that wanna be super-efficient programmers with only one question in their head: "Can I make it work a tad faster or smaller in anyway?"

So, during my search on the web few months ago, I came across a question on Stack Overflow, regarding for loop vs while loop on basis of performance. Since in that thread it was mentioned for loops are slower than while and loops with decrementing iterator are faster so I rewrote the code for better performance.

Here's the new version with for replaced with while and conditions edited for decrementing:

console.time("looping");
var gridSize = 5000, str = '', i = gridSize, j;
while (i--) {
  j = gridSize;
  while (j--) {
    if ((i - j) % 2 === 0)
      str += " ";
    else
      str += "#";
  }
  str += "\n";
}

//console.log(str);
console.timeEnd("looping");

But to my surprise this code had even worse performance. Then, after a while I found out that if ((i - j) % 2 === 0) was the main culprit, replacing minus sign with plus reduced the total time of execution to ~ 750ms

//Checked on NODE.js = v6.11.2
Book version of code         -->    893.76 ms
while loop with subtraction  -->    1562.43 ms
while loop with addition     -->    749.62 ms


//firefox Benchmark v54.0 (64-bit) (OS Ubuntu 16.04)
Book version of code         -->    725.10 ms
while loop with subtraction  -->    1565.29 ms
while loop with addition     -->    601.12 ms

Why is subtraction having such huge impact on total execution time?

Edit 01 6:30 AM (GMT) 8th August

After looking at @jaromandaX answer I'm Pretty sure that it is not the subtracting thats slowing down this loop, its the modulo of negative Number. Again I wanna know what makes modulo of negative number slower