I do believe there are speculative execution mitigations, yes...and from what I remember there was a significant performance impact when they were implemented,

I do believe there are speculative execution mitigations, yes...and from what I remember there was a significant performance impact when they were implemented, and especially so on processors without dedicated code caches. Syscall latency is definitely not the end-all-be-all of performance, but it is all there is to compare between a monolithic kernel and a microkernel. In both cases, driver code needs to run, and that driver code can be good or shitty just like everything else. When it comes down to it, the syscall latency that is comparable is the context switch between privileged and unprivileged mode. The holy grail of microkernels isn't to be faster than macrokernels, it is to be just as fast, but a hell of a lot more modular, usable, and secure.