Meltdown/Spectre: It's a vulnerability that has existed in all processors for the past 20 years. The fix isn't going to slow down your modern processors. Consumers should be aware of it, but it isn't some crazy imminent threat. There are no known exploits out in the wild at this time.
Windows 10 is already patched, as are most other devices. As always, just make sure you have your devices up-to-date. The patches aren't going to slow down your phone or modern PC. However, there have been issues with a previously released patch for older systems with AMD processors -- but Microsoft has already pulled the patch.
Here is an easy to follow explanation I got from a Cisco engineer:
"Modern processors get some of their performance by trying to guess what you’re going to ask it to do next. It predicts that you’re going to want the CPU to do a task, and gets started on it. So, by the time you ask it to do the work, it’s either already done or close to done.
The flaw is that researchers figured out how anyone can ask the processor what it has guessed you will want to do. This means that an attacker can read things in the computer’s memory that they’re not supposed to have access to. This affects places like Amazon’s EC2 or Microsoft’s Azure where they run virtual servers for many different customers on the same physical hardware."
I would have written an article on this, however, when you get into it, it's quite overwhelming, and it's already been explained so well by another trusted and experienced tech writer, Woody, who is all over the story. So, for more details and clarity, I highly recommend reading the following pieces from Computer World's Woody Leonhard: