Local network is WiFi, at least as far as the Deck is concerned. What it’s talking about is pulling the files from the already downloaded copy on your PC, versus downloading from Steam servers online.

Now, why it’s slower is unclear. I haven’t really paid attention honestly myself when downloading one way versus the other. I also don’t often have a game installed on both, anyways, because if it’s on my PC, it’s because I’m probably streaming it to my Deck.

That said, the download you get from Steam’s servers is heavily compressed, and then have take some period of time to decompress once you actually download everything. Meanwhile the files on your PC are already uncompressed, so it may be more, larger files, but it should still overall be faster. Maybe try installing both ways and see which total download time is actually taking longer. You might be surprised.

Some ISPs put a data cap every month and games are big af files, so steam made it where if a game is already installed on a local computer that has steam open it will try and copy the files over your local network. It could be slow if the other computer is being used or the drive it is reading the data from is slow.

You most probably have a normal pc/laptop that’s connected to the internet and with steam (and your games) installed. This means that steam doesn’t access their servers to download a game but instead copies the files from your other pc to the deck. This is usually faster and doesn’t really use your internet connection.

Just turn off local transfers if you only want it to download from steam servers. As for why, to keep data usage down. No sense downloading a 100gb game when there is already a copy in my house or saturating the WAN for the rest of the house while it downloads. Lots of folks have usage caps as well.

Our router supports 100 megabit per device only :'(