Well others may know more, but based on Microsoft's answer to a question I posed back around WinRT announcement time:
WinRT is a protocol and a set of Native APIs, allowing each language to remain true to its existing execution environment - Chakra for JavaScript, CLR for C# and the CRT/raw native code for C++.
That suggests similar performance trade-offs to what we currently experience using the CLR vs. native code to access what is essentially a native code API (WinRT). But I look forward to seeing some empirical investigations into this to see just how different WinRT is.
