It is, though in the vast majority of our uses that still likely makes for a difference of less than a millisecond, probably even without JIT (which Simon points out isn't available in translators, making everything about an order of magnitude slower).

There's certainly nothing wrong with using i++, since it's technically the correct thing to use and measurably (if imperceptibly) faster, but I'm just saying that we don't need to go and replace the thousands of existing uses of for...in in Zotero.