WPF : Focus delay when clicked away and back to the app

I have a datagrid inside the WPF application and I need to have it very responsive so that the user can work on one screen with Excel and on the other screen work with WPF application's datagrid.

When I click away from my WPF application to Excel and click back to the WPF application, there is a light half-second delay before the app is focused again.

What could be the cause for this delay? What can I do to reduce or eliminate this delay?

Your help on this is hi开发者_JS百科ghly appreciated,