If your internet is slow, reduce Zoom's bandwidth consumption: turn off HD video (Settings > Video > uncheck HD) — this alone drops video from ~3.8 Mbps to ~1 Mbps. Turn off your camera entirely during group calls when you don't need to be visible. Ask the host to stop incoming video for all participants. Disable virtual backgrounds and video filters, which consume extra CPU and bandwidth. In Settings > Video > Advanced, uncheck "Optimize video quality with de-noise." Close screen sharing when not actively presenting. For audio only: mute your video and ask others to do the same, which drastically reduces bandwidth to under 100 Kbps. Close other apps using bandwidth (cloud sync, streaming, Windows Update). Use the Zoom phone app on cellular data as a backup if your home internet is too slow.
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