It's the opposite way around. If you read it closely they say on windows 10, some customers may see a decrease in performance, but for windows 7, most customers will see a decrease.

An article in the verge states "Windows 7 and Windows 8 will be the worst hit simply because these older operating systems have features like kernel-level font rendering that will be impacted by the Spectre and Meltdown mitigations even further than Windows 10."