A user in our Discord said microsandbox felt slow. Listing every file in the Python standard library took 5.3 seconds inside a sandbox; in Docker it took milliseconds. We went digging. We fixed it in v0.4: we replaced our user-space filesystem with a Linux disk image that the VM mounts directly. The geometric mean speedup across our mixed guest-visible filesystem suite is 47×, with the worst-case rows more than 1,000× faster, and the host filesystem code is about 5,300 lines shorter. My first tr...