GroupBy implementation in Golang for string slices

GroupBy.go

func GroupBy(slice []string, groupFn func(string) string) map[string][]string {
	groups := make(map[string][]string)
	for _, elem := range slice {
		elementKey := groupFn(elem)
		existing, present := groups[elementKey]
		if !present {
			existing = []string{}
		}
		existing = append(existing, elem)
		groups[elementKey] = existing
	}
	return groups
}

@ashwanthkumar The locks are on the group rather than the entire map, so it essentially is wasting resources in the thread waiting for the lock to get free if it writes into the same group, but if they write to different groups it does get faster right ? Am I saying this right ?