readonly struct and readonly struct instance members (optimization?)

public readonly struct TestStruct
{
  public static TestStruct New()
    => new() { _array = new float[SomeEnumLenght] }; 
  private readonly float[] _array;
  public readonly float this[SomeEnum key] // IMO adding "readonly" to instance members when the hole struct is readonly is a bit silly, but just adding it here incase that makes a difference?
  {
    get => _array[(int)key];
    set => _array[(int)key] = value;
  }
}

static class Program
{
  static void Main()
  {
    var test = new TestStruct();
    _=test[0];
  }
}