Microsoft has warned that the fixes being rushed out to mitigate the risks from the Meltdown and Spectre vulnerabilities could "significantly" impact the performance of some systems.
In a blog post, Microsoft's Terry Myerson, executive vice president of the Windows and Devices Group, said that the company has already found that the patches are having an impact on performance - although it has yet to carry out extensive benchmarks.
For users running Windows 10 on a newer 2016-era PC with Skylake, Kabylake or newer, Microsoft's early benchmarks show single-digit slowdowns, which means users are unlikely to notice much of a difference.
However, anyone running the operating system on older silicon, such as 2015-era PCs with Haswell or older, can expect to see more "significant slowdowns", which means they will likely notice a degradation e in system performance.
For the majority of Microsoft customers - those running Windows 7 or Windows 8 - the firm "expects most users to notice a decrease in system performance".
It's even worse news for Windows Server customers, who'll typically be running their systems much harder.
Microsoft is advising users to "evaluate the risk of untrusted code for each Windows Server instance, and balance the security versus performance trade-off for your environment".
That is to say, users are being advised to consider whether they really need to install the patch, given the performance hit they're likely to suffer.
Myerson continues: "Windows Server on any silicon, especially in any IO-intensive application, shows a more significant performance impact when you enable the mitigations to isolate untrusted code within a Windows Server instance."
Microsoft's statement suggests that the slowdowns could be more substantial than Intel previously indicated.
Last week, the chipmaker said that these patches would not create any issues with computer slowdowns. However, Intel CEO Brian Krzanich backtracked somewhat during his CES trade show keynote on Monday, claiming that the impact will be "workload dependent".
"We believe the performance impact of these updates is highly workload dependent," Krzanich said.
"We expect some may have a larger impact than others, so we'll continue working with the industry to minimise the impact on those workloads over time.
Intel has said that it will patch "90 per cent" of affected processors made in the past five years by the end of this week, with the remaining 10 per cent to see fixes by the end of the month.
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