Depends on how you are viewing. If you are streaming jpeg images (as is default for UI2) then you could use the Low Quality mode:
The quality is then configurable in the UI2 Settings panel, Video Streaming category, at the bottom. You can also add a delay between jpeg refreshes to reduce bandwidth without reducing image quality. This option is at the top of the Video Streaming category.
If you are viewing H.264 (not the default for UI2) then you can simply adjust the bit rate setting in
Blue Iris for streaming profile 0. I'm not sure if you can change which streaming profile will be used. In my experience it is always encoder profile 0. But if you can change it, it will probably be in the bandwidth limiting options built in to Blue Iris in the Edit user dialog.