"Red Hat’s internal performance testing of a worst-case microbenchmark showed a significant slowdown. However, more realistic applications that utilize vector gathering showed only low single-digit percentage slowdowns."

https://access.redhat.com/solutions/7027704

Performance Impact

The performance impact of the microcode mitigation is limited to applications that use the gather instructions provided by Intel Advanced Vector Extensions (AVX2 and AVX-512) and the CLWB instruction. Actual performance impact will depend on how heavily an application uses those instructions. Red Hat’s internal performance testing of a worst-case microbenchmark showed a significant slowdown. However, more realistic applications that utilize vector gathering showed only low single-digit percentage slowdowns.

If the user decides to disable the mitigation after doing a thorough risk analysis (for example the system isn’t multi-tenant and doesn’t execute untrusted code), the user can disable the mitigation. After applying the microcode and kernel updates, the user can disable the mitigation by adding gather_data_samping=off to the kernel command line.Alternatively, to disable all CPU speculative execution mitigations, including GDS, use mitigations=off.

Is that 50% overhead for "Gather" instructions? If that is the case, then if 10% of instructions are "gathers" in your workload then that would be 5% overall.

> some workloads may experience up to 50% overhead

They word there are "some workloads", I suspect that performance hit is rare.

Key context you elided: "This depends on whether Gather is in the critical execution path of a program."

It is kind of like thinking that it is a huge sale at a store when they say "Up to 70% off". And there are like two things in the whole store which are 70% off.

I am always suspicious of "up to" claims.