CPUs tend to get slower with time. Probably not for the reason you’d expect.
Unfortunately, many microcode updates are security related, which almost universally cause performance regressions…or even complete feature loss!
Haswell might be the worst performing processor family of all time, in terms of features removed.
TSX - concurrency instruction literally removed due to security concerns
DX12 - removed / broken due to priv escalation
Meltdown, Spectre, Foreshadow, MDS - all lead to perf regressions…for MDS specifically Red Hat and Microsoft still suggest disabling HyperThreading on Haswell!
Over the life of a CPU, I’d expect the typical performance curve (from microcode) to look like:
Initial Launch to 1 year, slight upward trend, platform maturing
1-2 years, mostly flat
2 years, likely downward trend. Enough time has passed where security people are starting to poke/write papers indirectly leading to vendor microcode regressions
6 years / EOL -> old CPUs start to have major regressions!
I genuinely wonder if extremely performance-sensitive industries sometimes purposefully delay or otherwise remain on “stale” microcode…and just accept the additional risk.