[mraleph]

> At the same time, if I'm trying to simplify the test case and set only numeric values and not object ones, I've got nearly the same results: 3.953s and 3.922s for 10x more iterations (prop-benchmark-f.js and prop-benchmark-g.js).

Well, the secret here is that even if you have an empty constructor V8 still initially gives enough slack to you object for 10 in-object properties, and by coincidence you have precisely 10 properties.

Just add one more and observe the difference.