Apple Support:

Intel has disclosed vulnerabilities called Microarchitectural Data Sampling (MDS) that apply to desktop and notebook computers with Intel CPUs, including all modern Mac computers.

Although there are no known exploits affecting customers at the time of this writing, customers who believe their computer is at heightened risk of attack can use the Terminal app to enable an additional CPU instruction and disable hyper-threading processing technology, which provides full protection from these security issues.

This option is available for macOS Mojave, High Sierra and Sierra and may have a significant impact on the performance of your computer […]

Testing conducted by Apple in May 2019 showed as much as a 40 percent reduction in performance […]

You probably don’t need to enable these mitigations unless you’re a secret agent but I’m pretty sure this is really helping push the transition from Intel to ARM inside Apple.