Comment by comex

That document describes filesystems which are accessed over IPC, not filesystem-as-library like you seem to be describing. In fact, it's the same basic idea as FUSE. One user process (accessing the filesystem) makes an IPC call to another user process (server that implements the filesystem), which necessarily passes through the kernel and performs a context switch in each direction. On the other hand, it's quite possible that Fuschia's IPC is better optimized than FUSE, so it might have better performance in practice.