I would imagine browsers optimize the iterator protocol away for arrays. At least on https://jsben.ch/iJFZ4 for..of is a smidge faster than an old-school loop.
… but in a way you're correct that this is rarely the actual bottleneck in the surrounding code. Still, "death by a thousand papercuts" and all that. Plus having old-school for-loops as a habit makes it easier to spot the true botllenecks.
Also, for the record: I presume you tested on Chrome, but on Firefox the difference between the two loop styles is quite a bit larger (and generally slower than Chrome, so perhaps also a consideration when optimizing for "slowest browser" as the bottleneck).