Due to a design flaw that puts machines at risk. Lots more at the link. In short, because a flaw that could enable the machine to be hacked, all OSes which run on Intel chips made in the past ten years are getting a patch to fix this. As a result of the patch, the machines will run from 5 to 30% slower, depending upon the chip and the applications running. I'm so glad that I buy AMD chips and machines.