My coarse understanding is monolithic kernels historically gained favor over microkernels for practical performance reasons bound to the cost of context switchi

My coarse understanding is monolithic kernels historically gained favor over microkernels for practical performance reasons bound to the cost of context switching. I always felt there was opportunity to address that in order to have your cake and eat it too (eg. reserve registers for the kernel to avoid pop/push, maybe even dedicate a whole core and its state). How fast / lightweight is context switching between userspace and kernel code in Xous? Can you compile in such a way to avoid context switching altogether? What are the most significant overheads message-passing between services and where are is the lowest-hanging fruit for improving?