While Intel tries to downplay the performance hit of the software-based mitigation for the newly discovered MDS vulnerabilities in its processors, some vendors are painting a more dire picture. Apple for example has acknowledged that a small number of Mac owners may see performance drops of up to 40 percent due to the new patches. This only applies to full mitigation, which involves the disabling of Hyper-Threading.
A full mitigation could be applied, eliminating any possibility of the issue affecting a Mac, but in the process it disabled hyper-threading and, by Apple's estimates, reduce system performance by as much as 40%. This reduction only applied to anyone who enabled the full mitigation in the Mojave update, as well as those who installed Security Update 2019-003 for High Sierra and Sierra and similarly enabled it.
This potential loss of performance immediately caused uproar from concerned users, though the anger is overblown, and not specific to the Mac.
Windows users are also affected, but Microsoft doesn't recommend the disabling of Hyper-Threading due to the big performance impact. Basically, all these CPU vulnerabilities are a giant mess and it will take a couple more years until this is all patched on the hardware-level. This type of security has been neglected for too long, and now Intel and its users are bearing the brunt.