Remember Meltdown, Spectre CPU bugs from 3 years back? Those are now fixed by kernel modules. Obviously there's a performance impact. Someone recently measured the impact after 3 years and found for i7s these fixes reduce your CPU speed by 25%. Big impact still.
You can disable it by setting mitigations=off as kernel parameter while booting.
https://www.phoronix.com/scan.php?page=article&item=3-years-specmelt&num=9