如何使此代码更具性能？

I have this snippet of code that iterate over a map and filter some fields based on relation type, I have to run two loops and have a feeling that it is going to be slow for big maps. Is there any way I can technique/refactoring to make this code more performant.

func getFields(filter map[string]map[string]bool, msg *Message) (fs []Field) {

    for k, _ := range filter {
        if relationString(msg) == k {
            if fieldFilter, ok := filter[k]; ok {
                for _, f := range msg.Fields {
                    if _, ok := fieldFilter[f.Name]; ok {
                        fs = append(fs, f)
                    }
                }
            }
        }
    }
    return
}

You don't need the outer loop as one improvement:

func getFields(filter map[string]map[string]bool, msg *Message) (fs []Field) {
  if fieldFilter, ok := filter[relationString(msg)]; ok {
    for _, f := range msg.Fields {
        if _, ok := fieldFilter[f.Name]; ok {
            fs = append(fs, f)
        }
      }
    }
  }

  return
}