Where people seem to really care about perf is networking. What that means in non-crazy land is you cut out the kernel from that part, do networking in userspace, and pin a thread to a CPU to poll for packets. Like it's all MMIO and DMA anyway so ring 0 doesn't have to get involved if it doesn't want to. Then for other stuff you have convenient slow OS facilities. Including threads, processes, and disk I/O! Luxurious.
So it's not just that this has to perform better than a kernel, it has to perform better than not involving the kernel in the critical path, and it has to perform so much better that it's worth wanting to beat yourself in the face with a hammer after trying to diagnose the latest lockup. Unikernels are interesting to me, but like the demoscene or a semi tractor doing wheelies. Not for production usage.
So it's not just that this has to perform better than a kernel, it has to perform better than not involving the kernel in the critical path, and it has to perform so much better that it's worth wanting to beat yourself in the face with a hammer after trying to diagnose the latest lockup. Unikernels are interesting to me, but like the demoscene or a semi tractor doing wheelies. Not for production usage.