I actually benchmarked this and the perforance changes based on the number of keys.  For very small objects for in is better, for large .keys().map() has a slight edge:

```
Benchmark                                                            Samples      Time   GC Time        Per                Rate

Iteration
  Over the keys of an object with 10 members using for in            500,000    0.042s    0.000s    0.085µs       11,700,000/sec
  Over the keys of an object with 10 members using .keys.map         500,000    0.768s    0.000s    1.536µs          651,000/sec
  Over the keys of an object with 25 members using for in            200,000    0.631s    0.000s    3.155µs          316,000/sec
  Over the keys of an object with 25 members using .keys.map         200,000    0.640s    0.000s    3.202µs          312,000/sec
  Over the keys of an object with 50 members using for in            100,000    0.664s    0.000s    6.643µs          150,000/sec
  Over the keys of an object with 50 members using .keys.map         100,000    0.602s    0.000s    6.024µs          165,000/sec
  Over the keys of an object with 75 members using for in             66,600    0.642s    0.000s    9.635µs          103,000/sec
  Over the keys of an object with 75 members using .keys.map          66,600    0.564s    0.000s    8.461µs          118,000/sec
  Over the keys of an object with 100 members using for in            50,000    0.646s    0.000s   12.927µs           77,300/sec
  Over the keys of an object with 100 members using .keys.map         50,000    0.541s    0.000s   10.815µs           92,400/sec
  Over the keys of an object with 250 members using for in            20,000    0.665s    0.000s   33.226µs           30,000/sec
  Over the keys of an object with 250 members using .keys.map         20,000    0.514s    0.000s   25.691µs           38,900/sec
  Over the keys of an object with 500 members using for in            10,000    0.642s    0.000s   64.230µs           15,500/sec
  Over the keys of an object with 500 members using .keys.map         10,000    0.525s    0.000s   52.504µs           19,000/sec
  Over the keys of an object with 1000 members using for in            5,000    0.706s    0.000s  141.294µs            7,070/sec
  Over the keys of an object with 1000 members using .keys.map         5,000    0.557s    0.000s  111.314µs            8,980/sec
  Over the keys of an object with 10000 members using for in             500    0.789s    0.000s 1578.092µs              634/sec
  Over the keys of an object with 10000 members using .keys.map          500    0.604s    0.000s 1208.014µs              828/sec
```