Flags (keep in mind google can remove these options)
Disable chromecast : vivaldi://flags → disable Load Media Router Component Extension, Cast Media Route Provider, Connect to Cast devices on all IP addresses, Global media controls control Cast start/stop then relaunch Vivaldi
LiteVideos #enable-lite-video #lite-video-default-downlink-bandwidth-kbps #lite-video-force-override-decision : not a lot of info but might slow down every video / force 2 min pre-load limit