Discord seems rather good at picking data centers for chat channels optimized for good voice quality, but not always as good at picking data centers for video quality. There's some latency graphs you can discover "hidden" in Discord's desktop interface. (It's a hover over the right part of the call "bar".) You can use that information to manually override the data center for the channel. I've had some good experiences with switching data center to closer to the people experiencing the worst video quality. (Of course, you need to be a channel admin to manually change the data center. It will also kick everyone else and force a rejoin to connect to the new data center, so it is something to add to a tech rehearsal ahead of time if relying on the channel for important meetings.)