?? The layering unionfs (really aufs or overlayfs) mounts docker does are all in kernel. There's no userspace involvement in the actual file ops. I'm not at all

?? The layering unionfs (really aufs or overlayfs) mounts docker does are all in kernel. There's no userspace involvement in the actual file ops. I'm not at all sure what you think any of that has to do with the idea of usermode filesystem handling. Anyways, on linux userland filesystem stuff (ie. fuse) is slow because the kernel manages the vfs layer and has to mediate. In a system designed for non-privileged vfs it doesn't have to be anywhere near so involved. And beyond that, there's really nothing in particular that a filesystem driver does that needs to be privileged to 'run fast'. VFS is pretty much the posterchild case for how you can pull things out of a kernel and maintain performance, tbh. There's no reason at all it can't be fast except the kernel getting in the way rather than helping.