The good news: Microsoft is updating your PC to protect against a major microchip security flaw.

The bad news: The fix is going to slow down your computer.

Intel and other chipmakers revealed on January 3 that potentially serious vulnerabilities nicknamed Spectre and Meltdown could allow bad actors to access the stuff on your computer or phone. Microsoft has been working on a fix for months and hit the button on a Windows update after the flaw was made public last week.

Microsoft (MSFT) took the rare step Tuesday of detailing exactly how its latest security fix would impact your computer's performance -- down to the millisecond.

All Windows PCs will slow down to a degree. Windows 7 and 8 are installed on 51% of computers, according to NetMarketShare. Windows 10 is only on a quarter of PCs.

One- and two-year old PCs running Windows 10 will hardly be affected. Windows chief Terry Myerson said in a blog post that those computers will slow down by "single digit" percentages, resulting in PCs that run commands a few milliseconds slower. Customers will hardly notice the difference, Myersohn expects.

PCs made before 2016 will slow down more significantly. Myerson said some customers will notice that their PCs' performance has changed.

Most people who have PCs running Windows 8 and Windows 7 will notice that their PCs are running slower. That's because the older operating systems manage some software differently than the newer Windows 10. For example, all fonts in Windows 7 and 8 are rendered on the kernel, the software that controls the processor. The security update slows those kernel processes down.

Related: Intel CEO praises tech industry response to chip flaws

Windows customers aren't alone. Spectre and Meltdown security patches will also impact the performance of Macs, iPhones, iPads and other Apple (AAPL) devices. But Apple claimed the slowdown will be negligible -- there's "no measurable reduction in performance" on the operating systems and only as much as a 2.5% decrease in speed on the Safari web browser.

A Google (GOOGL) spokesman said the company is set to announce its performance expectations for Android's chip flaw remedy as soon as Wednesday.

Tech companies are very rarely this transparent about potential slowdowns to their software due to security updates. The difference is the Spectre and Meltdown flaws are the chip makers' faults, and not the software companies'.

The security fixes for the chip flaws haven't gone all that smoothly. For example, Microsoft's security update ended up rendering useless some PCs equipped with AMD chips, forcing them into a perpetual blue screen of death.