All particles will be processed, not only those on the screen. Those not on screen might be culled from being drawn, but it might still have an impact.
If you are worried about performance, it might be best to split the particles into chunks, maybe roughly screen-sized. So instead of having one box that covers the whole map, you would have many smaller boxes. Then, you can enable them based on proximity to the player (by manually checking the distance or by using an Area2D node).
