Many of the performance improvements that you’re talking about have been implemented a long time ago by NFS. While I understand that NFS certainly has it’s drawbacks, would it be possible to offer NFS as an alternative that users can switch to while the issues in osxfs are ironed out? Despite @eugenmayer’s assertion that NFS is too slow to be useful, I’m quite happy with it in most of my Vagrant environments. I work on large Drupal sites, so there’s some slowness, sure, but it’s certainly not intolerable, and I’d consider it fast compared to osxfs right now. No offense intended - I know you’re working on it - but that’s what I’ve observed.
More broadly, I’m curious about why NFS (or some other existing/proven project) wasn’t chosen for the base to build on here. If that were the case, the only custom bits that would need to be built would be the event propagation from host -> vm and maybe some caching trickery to speed things up in the VM.