The way the array is constructed at least leads to holey arrays in V8. Perhaps similar in other implementations. The performance characteristics between different array types may vary significantly as well. Still, the traditional for loop, counting upwards seems to be fastest for me across Chrome and Firefox on Linux. For-of is slowest by a wide margin.

There is always going to be a big gap between a browser that had a particular optimization, and one where it is absent.

But, browsers are an odd case where optimizations often prove not to be worth it on real world websites. The cost to optimize can easily exceed the savings.