The CPU/hardware related bugs surfacing the last couple of years have mostly been fixed by adjusting the software that is run. Sometimes only by disabling certain features of a CPU or patching the microcode in the CPU itself.
The issue with this is that by fixing these issues features got disabled and workarounds had been introduced that lowered performance. Dramatically so for some use-cases.
By how much? Well it really depends on your CPU and use-cases. But maybe you want to try yourself. If you want to know the most current parameters to pass to your kernel on boot-up to disable all the performance impacting fixes, go here:
It is not recommended to have this in productive use – as you can imagine. Those bugs where fixed for a reason.