What instance is used as a reference for the delay? One you self-host (lemmy.management)?

Yes. lemmy.management. It is purposefully updating subscribed communities to as many as possible (via automation.) This doesn't correct for network lag, but the idea was to capture the "federation" lag. There's no code I'm aware of that allows admins to prioritize outbound federation traffic. I could be wrong though.

Sooo … what’s the deal with lemmy.ml … that seems to have gone beyond lag and is basically falling over … seems like the devs have neglected their own instance’s health?

I just collect the data.

What’s that Redash? Is it a plotly thing or some other product that just uses their graphing library? How have you found it?

https://redash.io I don't remember how I found it. Probably an "awesome" list on github.