Everyone needs to chill out with the performance worries on this patch. We'll see once there's been more time for benchmarking, but initially it looks like performance impact will be small for must user workloads, since they largely stay in userspace and don't switch to kernel much, and probably less impact in Windows than Linux due to the differences in architecture. The people who have big issues are where they are doing virtualization, like cloud providers.
So unless you are running your system extremely close to maxing out, don't panic, just see how things go, you can always turn it off if you really need (there are powershell commands to control it now).
Also please note this is
NOT a Windows thing, all OSes are affected so don't think you don't need to patch OS-X or Linux. Alsoalso for the moment, AMD is affected in that the KPTI patches are applying to ALL processors for now, not just Intel. More research needs to be done before they decide to start applying it more selectively.