Steam let's you throttle downloads but apparently not uploads. Maxing out your upstream will make the pc feel that way. If you don't intend to resume games on a different pc, go into your library, right click factorio, hit properties, then updates. You can turn off sync there.

That's what I ...

I bought the game about a week ago on Steam and am really enjoying it.
However, every time I close the game and the sync (with I assume Steam cloud) starts, every other internet related thing gets fucked with, i.e. either being REALLY slow or not even responding at all. On top, that sync takes a ...