It's been clear for a while that the fixes for the Meltdown and Spectre memory vulnerabilities would slow down PCs, but just how bad is the hit, really? Microsoft has run some benchmarks, and it's unfortunately bad news if your system is less than fresh. While the patches for Meltdown and one variant of Spectre will have a "minimal performance impact," fixing a second Spectre variant through low-level microcode imposes a tangible speed penalty -- and it's particularly bad on systems released around 2015 or earlier.
If you have a PC with a 6th-generation Intel Core processor or later, you should only see a hit in the "single-digit" percentage range. That's small enough that you might not notice a thing. If you're using a 4th-generation Core or older CPU, however, you'll see "more significant slowdowns," some of which could be noticeable. And that's if you're using Windows 10. Most Windows 7 or 8 users on similar hardware should notice the effect, while Windows Server is bound to take a more substantial performance blow due to its heavy dependence on I/O.