Big picture for virtualization? There's many, it's a complex emerging phenomenon which placed a lot of asterisks on computation, is it used for security? Or to patch tech debt? or to provide backwards compatibility? Yes. What is the limit between isolation and efficiency? No one knows yet and there's many answers, you can activate shared cpu caching or disable Spectre mitigations for efficiency, or you can just forego virtualization as a whole and depend on application level multitenancy, which is not a separate topic at all in terms of virtualization, as you can see with tech like containers, and t1/t2 hybrids, it's a spectrum of virtualization, there is no hard line separating the virtualized from the bare metal.