No, because accessing a object property requires a hash table lookup.
In other words, it's not very helpful to say "this is so slow, you should do this instead" without at least trying to quantify the difference somehow.
With 10M entries in the array, the "slow" way takes 509 ms and the "faster" way takes 362 ms. So we can save roughly 15 nanoseconds per iteration. Or a 30% speedup - assuming nothing happens inside the loop.
(Of course the numbers may vary wildly per browser.)
Somehow I doubt this is the performance problem behind slow web pages.