How about an immediate-mode renderer that works like a Rust async executor?

The executor could run all of the rendering closures on one core and benefit from caching.

It could detect slow rendering closures and automatically switch them to buffered mode and run them on a "slow renderers" thread. This would keep the rest of the UI snappy. It could also tint those portions of the UI so users can see which app is the culprit of UI slowness.

I hope that Rust can someday be used as the basis of an OS that does not rely on sandboxing for security, but instead uses the compiler to enforce security. Such an OS could use cooperative multitasking and use a lot less energy and less expensive hardware than our current multi-core CPU + MMU + kernel ring model.