Irksome thing: undiagnosable lag.
Just yesterday, on an online server (running direwolf20), I stepped into the Twilight Forest. Things seems a bit glitchy, so i pressed <tab> - say an 'all green' ping meter. I typed /tps and nothing happened for fully 30s. Then I got a report indicating 14tps - not great, but nothing that could explain why I had an effective >30s lag that would not go away - despite the green server ping. After disconnecting and reconnecting several times, each time returning to massive lag I gave up for ~45 minutes. Then I logged back in, did a /tps and got an immediate report of 14ms. lag, mysteriously, gone.
Interesting.
I've seen something like this. Massive lags, long delays before getting responses, etc.
We tracked it down. I was working with Draco on testing for Reasonable Realism. It was tracked down to the behavior of volatile gas. And we found a rather strange bug in his code, and a bug in loaded chunk detection.
He had implemented code to let gas flow through "hollow" blocks, like open doors, or fence posts. Well, fence posts occur in mineshafts. He didn't check if there was solid blocks past that, so once a mineshaft had gas, gas would flow through solid stone (that was his bug). But he was constantly checking the chunks he was looking at to see if they were loaded, and they were constantly returning true.
We found very, very long, single-chunk wide sections of created terrain, sometimes going over 1,000 chunks. Because, in 1710, caves are less frequent -- and now it's possible to go that far before finding a cave in that straight line.
Apparently, chunks always returning "loaded" isn't just us -- Reika's had that same problem with reactors (his reactor tiles get ticked, can't tell unloaded, so generate a fission entity ... which sits and piles up).
Draco solved this, and the massive problems with gas delays went bye-bye.
Ping times, as far as I can tell, don't require a round-trip through the server's processing queue, but is handled by the network layer. So ... it's a low-level, "probably is not <synonym for useful beginning with g>" number