This is really bad news. I'm running a Core 2 Duo that is doing fine but is obviously not a power horse compared to newer CPUs and I'm concerned that even a 20% hit would be problematic. I'll have to wait and see.
I guess I can isolate my music machine from the internet so that I don't ever have the patch installed, but I really hate to do that.
I'm also curious if this affects my Windows tablets running Atom processors. They are adequate as-is but a 30 percent hit could be catastrophic. Lets not forget that this affects battery life, not just performance. It may be a very good thing that MS is investing strongly in ARM processors. That may turn out to be one of the best moves MS ever made.