Could you please advise how can I configure decoding settings to lower network bandwidth from Xeoma server to client in all cameras view mode?

Here are the ways to reduce traffic consumption between server and client:

1) On the server side go to Main menu -> Remote acces -> Users – and tick Forced video decoding on the client – this way a compressed stream will go over the network from the server to the client.
2) On the client side use tabs to distribute the cameras – the network is loaded only from those cameras that are currently on the screen, so several tabs with fewer cameras consume less traffic than all cameras on the screen at the same time.
3) On the client side go to the Layouts meny (squares icon on the lower panel) -> Client decoding settings – set Maximum number of cameras for which a high resolution stream will be displayed to 0 – so if there are several cameras on the screen, only the preview stream will be transmitted to the client, it usually consumes less traffic than the archive (higher resolution) stream.
4) (if you don’t use the 1st paragraph) On the client side go to the Layouts menu -> Client decoding settings – choose “Enabled” in Video decoding on the client side for live preview.
5) On the camera’s side lower 2nd stream parameters (resolution, FPS, bit rate). If you’re using MJPEG for the 2nd stream, then change it to H.264/H.265.
6) Use more effective compression on the cameras (e.g. H.265/H.265+/H266 instead of H.264).