Traps system calls and emulates them using Xok primitives. The key here is that you can do this at user-level without violating protection. Run a collection of applications and measure total throughput. Exposing the hardware makes it easier to add system optimizations. Downside of low-level interfaces means more recompiling for important applications.
Time slices are a simple, powerful way to share the processor.
Poorly chosen abstractions lose informaton. Microbenchmark performance is not necessarily important. Turning off interrupts for critical sections can be a performance win. Putting page tables at user-level is tricky. Not clear how useful downloadable interrupt handlers are.