What this measures
After a fetch, reads responseStart minus requestStart from resource timing when the browser exposes it. If the split is hidden, the page says so instead of guessing.
How to read it
Separate the calculators from the live checks. A calculator never opens a connection. A live check only talks to this website, and the byte count is part of the result.
What it cannot do
There is no upload endpoint and no packet-loss stream. A fast download of a small file can finish before the link is full. The path is only as far as this website.
Questions
Is the download test my ISP speed?
No. It times a file from this website. Distance, the sample size, and the host all sit in the number.
Why is there no upload reading?
A static site cannot accept an upload. The calculator only does the arithmetic you type.
Where is packet loss?
It is not shown, on purpose. A page cannot see dropped packets without a server that counts them.