The kernel module is essentially an interceptor, much like FUSE; the serious logic is still all in user space. When I talked to the developers about four years ago, they had considered using FUSE on several occasions but it had never performed as well - for them - as what they had already developed themselves.

When Linus says that user-space filesystems are just toys, he's simply wrong. If he were to say the same thing about FUSE in particular he'd be closer to mark, but a large part of the reason FUSE has lagged is Linus. You don't get credit for predicting an outcome you control, in my book. It's worse than it needs to be because Al Viro and others actively resist any changes that would make it better. Linus, who delegated this part of the kernel to others nearly twenty years ago, backs them up. Making the VFS layer look more exactly like what XFS or even NFS needs is par for the course. Making the smallest concession to what FUSE needs is verboten. I guess some folks are afraid of losing their niche if those artificial obstacles were removed so that more people could build filesystems without begging for keys from the gatekeepers.