the [...].includes?(o) is also pretty slow because at worst case it has to scan the whole array O(N). again, its possible to refactor this using a hash for O(1):
```
SOME_FIELDS = {
  'host' => true,
  '@timestamp' => true, 
  'plugin' => true,
  'plugin_instance' => true,
}
...
collectd.delete(k) unless SOME_FIELD.has_key?(k)
```