WinRT XAML is much faster than WPF ever was, it's just that they've started using it in very performance sensitive places where they never used any form of XAML

WinRT XAML is much faster than WPF ever was, it's just that they've started using it in very performance sensitive places where they never used any form of XAML before, like the calculator and start menu (the XAML calculator definitely doesn't take several seconds for me BTW, more like one second, but that's still too slow IMO), and it's still slower than the internal frameworks and bare Win32 calls they used to use. If they were WPF they'd be even slower.