Every time there is a major update, like a feature update, this question again springs to my mind.
In the Windows Update Advanced Options there is Delivery Optimization, which allows you to specify that parts of previously downloaded updates can be "fetched" from other PCs either on your LAN or from the world at large. I long ago restricted that to other PCs on my LAN, but I have yet to see evidence that such fetching has occurred.
In the case of major feature updates it could save many hours of download time if the first PC on the LAN that downloaded the update would be used as the source for same for all others on the LAN, but that never seems to happen.
Does anyone know if this mechanism is actually functioning, or is it currently a "place holder" for something that's planned for Win10 that is not yet implemented? It's been around in settings for a very long time, but all of my machines have consistently downloaded from Microsoft's servers independently of each other even if one machine on the LAN has gone through the complete update process already. If two or more go after a feature update download at once everything slows to a crawl, bandwidth-wise, since I have DSL.