https://www.theverge.com/2018/1/9/16868290/microsoft-meltdown-spectre-firmware-updates-pc-slowdown

Microsoft is taking the surprise step of detailing how Spectre and Meltdown firmware updates may affect PC performance. The tech industry has been scrambling to issue updates to protect against the two CPU security flaws over the past week, and there have been many reports of potential performance issues. The good news is that for modern PCs running Windows 10, most consumers won’t notice a significant difference. If you’re on an older machine, particularly a Windows 7 or Windows 8 one, then there’s going to be some noticeable performance changes.

According to Microsoft, Intel Haswell processors and older will be impacted the most by a series of firmware updates designed to protect against the Spectre CPU security flaw. Intel has been working with PC makers to ready firmware updates, but it’s fair to say most machines do not have these installed just yet. These updates will impact PC performance, but the level of impact depends on what that PC is doing and how old it is. Microsoft warns that most benchmarks we’ve seen reported “do not include both OS and silicon updates.”