Download time is -

When looking at the chart, the content download is -. If you look at the waterfall you see Green TTFB but no bar to indicated Content Download(Blue Bar).

When you look at the request Detail for the same item, you see a TTFB but the time for Content download is -.

That really just means 0ms. i.e. all of the data was read in under 1ms. You will see that if the response didn’t require additional round trips (under 4KB or so for the first request, more for later ones as the connection warms up).