Kast is designed to be low impact. Being in a Party consumes a variable amount of upload and download bandwidth depending on how many videos you are viewing simultaneously and if you are broadcasting video. Videos that are turned off do NOT consume bandwidth. Each video stream consumes 2 mb/s so if you are watching 4 videos and broadcasting at the same time you are using approximately 8 mb/s of download bandwidth and 2 mb/s of upload bandwidth.
Audio streams on the other hand are much smaller and occupy approximately 100 kb/s per stream both uploading and downloading.