The steady low line to the left of the graph is it using the “local network” from my computer (didn’t even know it had one) and it goes straight up when I interrupt it and tell to switch to the WiFi for download. And it keeps doing it so I have to switch the the WiFi manually every time. Why is that so and is there a way to “always use WiFi?”
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.