Adding "immutable" means that the browser should not even check the ETag if those files are cached. That saves a network connection and response round trip. That should reduce the load on the webserver some, but only really number of active TCP connections, rather than bandwidth, since the files were already being cached. This probably will only reduce the peak load a little bit since this doesn't affect index and APK downloads, but it will make page loads more responsive during peak load times.
Based on this thread with @Joost: https://forum.f-droid.org/t/slow-webserver-response/6968/15