I suppose JS can run faster in 'strict' mode

[ghost]

Quick check with just a laptop browser shows 202-205ms instead of ~1300ms

"use strict" // is really more perfomant
console.time("test")
var rounds = 10e7 // var is more natural = faster

var x = 1
var pi = 1

for (var i = 2; i < rounds + 2; i++) {
    x *= -1
    pi += x / (2 * i - 1)
}

pi *= 4
console.timeEnd("test")