Use findViewById in all scopes or use a variable?

Which of the following strategies is best suited for good application performance? Use findViewById whenever we use any element of View or do it only once by assigning to a variable?

example in Xamarin.Android :

public class MainActivity : Activity 
{

  private EditText _CampoNome;

  protected override void OnCreate(Bundle savedInstanceState)
  {
     ...

     _CampoNome = FindViewById<EditText>(Resource.Id.edtCampoNome);
     _CampoNome.Text = "ola mundo";

     EscreverNome(); 
  }

  private void EscreverNome()
  {
      Console.WriteLine(_CampoNome.Text);
  } 
}

or

public class MainActivity : Activity 
{
  protected override void OnCreate(Bundle savedInstanceState)
  {
     ...

     FindViewById<EditText>(Resource.Id.edtCampoNome).Text = "ola mundo";

     EscreverNome(); 
  }

  private void EscreverNome()
  {
      Console.WriteLine(FindViewById<EditText>(Resource.Id.edtCampoNome).Text);
  } 
}