Why Your SFTP Transfer Is Stuck at 2 MB/s (and the Fix Is a Protocol from 1983)

Two minutes to copy a 274 MB file to a VM running on localhost. Not over the internet. Not to a cloud instance across the country. Localhost. The same machine, loopback, zero network latency. That was the experience a user reported in issue #290 on cubic, a lightweight CLI for managing QEMU/KVM virtual machines. The maintainer reproduced it, traced the problem to the upstream russh-sftp crate, and posted a comment asking if anyone had ideas about where the bottleneck was. I did. The answer turned out to be a protocol design decision that limits every Rust project using this crate to about 2 MB/s on file transfers, regardless of how fast the link is. The fix was to stop using SFTP entirely and fall back to a simpler, older protocol. What Is cubic? cubic is a CLI tool for creating and managing lightweight virtual machines on Linux and macOS. Think of it as the middle ground between running Docker containers and spinning up full VMs in libvirt. You run cubic create myvm --image debian and