I run software that (thanks to poor design, IMO) manages to bottleneck context switching on Linux's monolithic kernel. I can't imagine this not being worse with more of the OS pushed out into userspace, though I'd love to be proven wrong!
The Mill CPU design doesn't have a user/kernel mode split, and it's also single-address-space (still with the same level of protection, just a different mechanism). Protection domain switching is done through portal calls, which are essentially shared library-like indirect calls with metadata for the CPU. Further, saving and restoring things like registers is done asynchronously. That should remove any remaining performance difference (and make monolithic designs faster too).
Their forum has some good responses from the team on follow up questions asked by the community as well.
It can potentially be even better, since you can potentially get more direct access to whatever resource you're contending on.