From the list of available audio codecs, deselect all except ``opus/48000''.
From the list of available video codecs, deselect all except ``VP8''.

You can make these changes via the interface of Jitsi:

  -> Jitsi-Tools-Options-Audio-Encodings
  -> Jitsi-Tools-Options-Video-Encodings

It is recommended that all the participants of an audio/video call use the
same codecs.