Should I declare converters in App.xaml or as a per-file resource?

When declaring converters in a WPF application, should I:

1. Declare all my converters in the App.xaml (i.e. in <Application.Resources/>) so it's available to the entire application
2. Declare only needed converters for each Page/Window/ResourceDictionary/UserControl etc. in their Resources section
3. Something else entirely

Regarding readability, method 1 seems the best to me, but my question is about performance. Which method is the most resource efficient in terms of performance, memory, etc.?

Well, I just don't declare them in xaml at all. Instead, I additionally derive a converter of mine from MarkupExtension. Like this:

public class MyValueConverter : MarkupExtension, IValueConverter
{
    private static MyValueConverter _converter = null;
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        if (_converter == null) _converter = new MyValueConverter();    
        return _converter;
    }

    public object Convert
     (object value, Type targetType, object parameter, CultureInfo culture) { }
    public object ConvertBack
     (object value, Type targetType, object parameter, CultureInfo culture) { }
}

This allows me to use my converter anywhere, like this:

Source="{Binding myValue, Converter={converters:MyValueConverter}}"

where converters is the namespace in which I have declared my converter.

Learned this trick from an old stackoverflow thread only.