Only ever done one or two US-UK links. Very good results IIRC.

When Jitsi (WebRTC) struggles it seems to do a sensible thing and fall back to preserving good audio but temporarily dropping the video completely.

Since it is browser based (webrtc), the implementation is mostly browser based. You can expect the same audio/video as you get with google meet or teams, which are using the same tech.

One or one calls should be using direct peer to peer connection thanks to webrtc.

Group calls will be routed by the server, so latency will be an issue depending on the server location.