What this measures
Pipes the bytes through CompressionStream gzip. You get the packed size, the ratio, and megabytes per second. Unsupported browsers get a clear message.
How to read it
Prefer the generated sample when you want a repeatable number. A file from disk adds the browser's read path. Hashing and compression start after the bytes are in memory.
What it cannot do
Caching, the browser, and the file picker sit in front of the drive. localStorage and IndexedDB are browser stores. They will not show a drive's sustained sequential speed.
Questions
Does file read speed equal my SSD?
No. The browser, caching, and the way the file is handed to the page all change the rate.
Will the localStorage test wipe my runs?
No. It uses a temporary prefix and deletes those keys. Saved runs live under a different key.
What if compression is missing?
The page says the streams are unsupported. It does not substitute a made-up rate.