Earlier quoted context omitted.
I wonder how hard it would be to replace FUSE with userspace block driver. That seems to be focused on performance. https://lwn.net/Articles/903855/
Isn't that implementing a block device, so the level below the filesystem?
Yeah. That said, I've become disillusioned with FUSE over the course of writing this. FUSE is... not great, it's just what we're stuck with. Suggestions as to alternatives to FUSE would be welcome.