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. Each video stream consumes 3 mb/s so if you are watching 4 videos and broadcasting at the same time you are using approximately 12 mb/s of download bandwidth and 3 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.