The cost of a switch is non-zero. For IPC you need to switch out the process running in the CPU, for Syscalls to drivers a microkernel will have to switch into priv ring, then out, wait for the driver, then back in and back out, as it switches context.
A monolithic, hybrid or modular kernel can significantly reduce this overhead while still being able to employ the same methods to amortize the cost that exists.
A microkernel is by nature incapable of being more efficient than a monolithic kernel. That is true as long as switching processes or going into priv has a non-zero cost.
The easy escape hatch is to allow a microkernel to run processes in priv ring and in the kernel address so the kernel doesn't have to switch out any page tables or switch privs any more than necessary while retaining the ability to somewhat control and isolate the module (with some PT trickery you can prevent the module from corrupting memory due to bugs or malware)
A monolithic, hybrid or modular kernel can significantly reduce this overhead while still being able to employ the same methods to amortize the cost that exists.
A microkernel is by nature incapable of being more efficient than a monolithic kernel. That is true as long as switching processes or going into priv has a non-zero cost.
The easy escape hatch is to allow a microkernel to run processes in priv ring and in the kernel address so the kernel doesn't have to switch out any page tables or switch privs any more than necessary while retaining the ability to somewhat control and isolate the module (with some PT trickery you can prevent the module from corrupting memory due to bugs or malware)