True, but the biggest problem .NET developers have faced is not performance, it's that there is a massive delay between new technologies becoming available in Windows and suitable quality Managed Wrappers being in place (the Ribbon being a classic example, but there are plenty of others). And developing your own wrappers is both time consuming and potentially problematic if "official" libraries come out in future that wrap things differently. C++ developers have suffered similarly, but it's considerably easier to just call C style APIs there.
WinRT brings a standard method for exposing new APIs in Windows that can easily be consumed from whichever platform suits the developer. Long term this has the option of makingn .NET (and potentially JS) much more of a first-class development platform than it has ever been before.