Really interesting to see how some chunks lag more or less than would be indicated by geography alone. For example, the Chicago and Indianapolis areas seem to be much more affected from the northwest than the southeast. I wonder if this is mostly due to topology or just grid capacity.

Do you know how the power plant's control loops are tuned? It seems like just tuning your plant to be properly damped isn't enough, because there's also lots of feedback from other power plants.

I don't know for a fact how they manage it, but I bet the feedback is slow enough (those generators are heavy, and it probably takes a few cycles to even accurately measure a change) that they might not have to do much in the way of actively managing that feedback. The system will oscillate a bit (as it did in that video), but there's nothing inherently wrong with that, as long as it's within a certain set of limits.

(And really, the load changes are typically more gradual. Even "everyone just got home from work" is a fairly spread out event, compared to, say, the sudden loss of a few hundred MW of generating capacity...)

Break time in big sport events also leave there fingerprints in the grid frequency. Everybody start to make a coffee, go to toilet (light), etc.