Crossing from usermode to kernel is slower, and generally doing complex things in kernel is much slower. (Where complex means cross-cutting concerns, so multiple different kernel processes need to be involved.)
Are there use-cases where a micro-kernel is just as fast as a monolithic?
Anything that doesn't really involve the kernel much at all, and things that take a lot of time in kernel but do so in one part of it with no need for input from other parts.
Generally, when you go for a microkernel you need to accept that it will be slower than monolithic. It might still be worth it for the advantages of microkernels, namely being easier to verify, understand and secure.
Comments
Crossing from usermode to kernel is slower, and generally doing complex things in kernel is much slower. (Where complex means cross-cutting concerns, so multiple different kernel processes need to be involved.)
Anything that doesn't really involve the kernel much at all, and things that take a lot of time in kernel but do so in one part of it with no need for input from other parts.
Generally, when you go for a microkernel you need to accept that it will be slower than monolithic. It might still be worth it for the advantages of microkernels, namely being easier to verify, understand and secure.