I think this would be the most usable microkernel these days, and yes, it does FS as a service: https://www.redox-os.org/

There's no inherent reason why a filesystem in userspace has to have a measurable performance impact over in-kernel, done right. It would be a lot of engineering work, though.

Context switches aren't that expensive when you're staying on the same core, it's IPIs (waking up a thread on another CPU) that's inherently expensive; and you need zero copy to/from the pagecache, which is tricky. But it totally could be done.