Apple has advised users that they should disable Intel's Hyper-Threading feature on the company's computers due to the recently exposed MDS vulnerabilities. Citing internal testing, Apple said that users can expect an up to 40% performance loss in such a scenario (depending on system and workload, naturally) in various benchmarks and multithreaded workloads. The performance loss is understandable - you're essentially halving the number of threads available for your CPU to process data.
Like Intel said, it becomes an issue of how much users value their performance compared to the security risks involved: a classic risk/benefit scenario, which shouldn't ever be in the equation, after all. If users buy a system with a CPU that has known performance levels, they will obviously expect those to be valid for the longevity of the product, unless otherwise stated and considering operational variances that fall within a margin of error/product obsolescence. Halving your performance because of a design flaw that resulted from an effort to achieve higher and higher IPC increases doesn't strike as a way to inspire confidence in your products.