Depending on how you use your computer, you may not even notice a difference. Based on Intel's benchmark results, though, you will notice some slowdown if you browse the web and use applications, and it's safe to say that most people do. Obviously, if you use your computer for heavy applications, the slowdown will be more noticeable. As Intel explained:

"In certain cases, some users may see a more noticeable impact. For instance, users who use web applications that involve complex JavaScript operations may see a somewhat higher impact (up to 10 percent based on our initial measurements)."

A few days ago, Intel admitted that the fix for the flaws that leave its chips open to remote attackers will slow down PCs, but it has changed its wording a few times when it comes to discussing the patch's performance impact. These numbers can give you a more concrete idea of how much you'll be affected.

In addition to releasing its benchmark results for PCs, Intel said it plans to share initial data on its assessment of server platforms these next few days. It also insisted that it has yet to receive any information that the Spectre/Meltdown flaw has been used to obtain customer data, but it's probably for the best to install the fix when you get it anyway.