I wonder how much of Intel's long-term perceived lead over AMD in performance is explained by compromising on these security issues that are only in the past few years being (known to be) exploited.
This might be true in recent history, but Intel's Core architecture had AMD on its heels for several years.
You have to distinguish between mitigating an issue in software/firmware, and fixing it in hardware. You can't unfab a chip, so the software/firmware mitigation is necessarily crude. The actual performance advantage Intel was getting from "cheating" was most likely small.