The meta store is a bottleneck too. For a shared mount, you've got a bunch of clients sharing a metadata store that lives in the cloud somewhere. They do a lot of aggressive metadata caching. It's still surprisingly slow at times.
I want to go ahead and nominate this for the understatement of the year. I expect that 2026 is going to be filled with people finding this out the hard way as they pivot towards FUSE for agents.
If you're running a FUSE adapter provided by a third party (Mountpoint, GCS FUSE), odds are that you aren't going to get great performance because it's going to have to run across a network super far away to work with your data. To improve performance, these adapters need to be sure to set fiddly settings (like using Kernel-side writeback caching) to avoid the penalty of hitting the disk for operations like write.
If you're trying to write a FUSE adapter, it's up to you to implement as much of the POSIX spec that you need for the programs that you want to run. The requirements per-program are often surprising. Want to run "git clone", then you need to support the ability to unlink a file from the file system and keep its data around. Want to run "vim", you need the ability to do renames and hard links. All of this work needs to happen in-memory in order to get the performance that applications expect from their file system, which often isn't how these things are built.
Regarding agents in particular, I'm hopeful that someone (which is quite possibly us), builds a FUSE-as-a-service primitive that's simple enough to use that the vast majority of developers don't have to worry about these things.
Those seem like pretty basic POSIX filesystem features to be fair. Awkward, sure... there's also awkwardness like symlinks, file locking, sticky bits and so on. But these are just things you have to implement. Are there gotchas that are inherent to FUSE itself rather than FUSE implementations?
My expectations are that in 2026 we will see more and more developers attempt to build custom FUSE file systems and then run into the long tail of compatibility pain.
How does that work with multiple clients though?