Run a for each loop only for the objects actively on the screen?

In the event line that executes the foreach loop, add a condition that limits the scope. You could try:
- wall x pos > x left frame
- wall x pos < x right frame

Glad I could help. If this is strictly for the player to test collisions, you should try using a "Is overlapping" condition to reduce the scope of the foreach loop. ForEach will only run enough loops to cover the the objects that meet the event's conditions. So in the suggestion I made, only objects that are positioned between the left and right of the screen will be checked int he loop. But you can also add conditions for walls inside the y positions of the screen (between y top frame and y bottom frame). My original assumption was you only moved left/right so I didn't include the y position check. But ultimately, if this is simply checking a couple objects against the walls and floors, you should use an overlap check to execute the loop. That way it is only running a couple loops every frame.