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

These changes can be performed from the main menu of Jitsi:
 * Tools > Options > Audio > Encodings
 * Tools > Options > Video > Encodings

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