We chased an upload running 8x below what the link could do, shipped three fixes that turned out to fix nothing, and found a real one that turned out to be far smaller than it looked — because the thing measuring the bug was also causing it.
The crime
I work on Coffer, a file store that encrypts everything in the browser before it uploads. Files go up in chunks: at the time of this investigation, a 4.8 GB file became about 300 sequential PUTs of 16 MiB, each one AES-256-GCM encrypted client-side before it leaves the tab. The server is ASP.NET Core, sitting on the same gigabit switch as the machine doing the testing, with the CDN entirely out of the path. Exact versions for everything below are in an appendix at the end.
Direction
Throughput
