I didn't believe the results and created new benchmarks for this: 
```
  bench('just set', function() {
    map[key] = value
  })
  bench('toString() > set', function() {
    var skey = key.toString()
    map[skey] = value
  })
```
The latter is actually nearly 30% faster on 1.11 and ~5-10% faster on 1.10. Dafuq?