Depends on which memory safe language they use and how (plus this blog post isn't by the Windows team or about Windows specifically). The biggest thing that slows Windows is lock contention, back-compat, and abstraction layers. Those problems all remain regardless of language choice.

Taking what's probably a joke too seriously for a moment, I don't think that follows at all. This is a research group within Microsoft doing exploratory work, so it shouldn't affect Windows at all. Further, they say they are "starting with Rust", which is not a whole lot slower than C++ (and you can always dip down into a different language for extremely performance sensitive parts).

Windows' slowness is almost all attributable to process startup time and a lot of modern apps being based on Electron, Chrome, JavaScript, and other very process-happy environments. Writing applications in Rust is not going to change Windows' process startup system.

First step was to keep C support around for legacy reasons and push C++ for systems level programming.

I did not notice any slowdown on a 10 years old laptop running the latest release from Windows 10, on the contrary it is quite snappy.