WPF uses DirectX for rendering (which is much faster) instead of the native Windows GDI.
As you might already know, Native GDI is bitmap based. As WPF is vector based, WPF can make use of hardware support, in a much better way when compared to GDI.
How ever, the decision to choose WPF or Winforms heavily depends on other factors as well - The flexibility you need in the UI, how much media support your application needs etc. WPF is not a replacement for a Winforms - and it is not a silver bullet for all the problems.
With more hardware acceleration features coming up, WPF can deliver much better than your Winform applications.
A couple of interesting blogs I read earlier on this