We routinely rely on virtualization and containers for isolation. Even in this very browser you're using, you are supposed to expect untrusted hostile code to run in other tabs. Imagine going to a website and their JS is reading memory from your password mananger, and would you then have a similar reaction?
Why do you need single core performance so badly. I can't even think of an example where single core performance has been an issue for me, and I routinely run tasks on decade old CPUs. And even if it was an issue, You are essentially saying security should be an after-thought,sorry but your reasonig is very dangerous. Would you get in a car where the engineer of the car thinks "if people are ramming your car on the freeway,you have bigger problems,let's focus on making it lightweight,fast and fuel efficient".
And to be frank with you, people that have been doing admin/engineering work since the 90's with that attitude are a bigger security threat to most orgs (and themselves) than any hacker (with the exception of the few orgs/people that receive targeted attacks frequently). The days of treating security as a perimeter issue have been long gone for about a decade now. Whether it is network, system or software security,the entry points and perimeters have been rendered meaningless (lookup zero trust, I think it applied here too).