[ballenf]

A trivial example is that a 'for' loop vs. 'while' loop can be faster on one browser compared to another.

These differences are even starker if you use newer features without polyfills. For example, 'forEach' implementations as of a year ago varied by orders of magnitude.

Native array sorting is another obvious one. Chrome, Firefox and especially Safari use quite different algorithms each based on array size with varying results.

If you add up all those little changes and always go with the option that's fastest in Chromium, in a reasonably complex SPA you could easily generate noticeable differences.

[cortesoft]

If 'for' and 'while' loops vary in speed that much between browsers, isn't that a defect in the browser? If one way is faster in chrome than firefox, why doesn't firefox change how it is doing the slower version?

[ballenf]

Because that would likely mean slowing down a different scenario. It's usually a trade-off, not a "defect".

The array sorting is a great example where the length of the array chosen or the pre-sort ordering can affect which browser is faster. No single browser may be able to claim the fastest for every scenario.