Apple has published an official support document regarding the enabling of full mitigation for Microarchitectural Data Sampling (MDS) vulnerabilities, otherwise known as ZombieLoad. Complete mitigation, which requires that hyper-threading be disabled, results in “a 40 percent reduction in performance with tests that include multithreaded workloads and public benchmarks.”
That is a severe loss, but some have pointed out full mitigation isn’t necessary for most folks. AppleInsider opinions that “unless the Mac is being used for highly secretive tasks, the user is a potential subject for hacking attempts by a sophisticated bad actor, or some other value-based reason, there isn’t really a need to turn on the full mitigation.”