Tested your cap out with Tulimide in chat, and we found a solution, as well as a bug.
It seems you were constantly adding the boundaries to the los obstacles, and thats most probably what was causing the fps drop.
So if you look at event 28 you'll see the always event, just change that to a trigger once while true, and drag your sub-event on its own. It doesn't need the always.
Now that will fix the fps, but Tulimide found a real bug with los picking multiple instances, and while it doesn't seem to harm the preview it drives the debugger crazy, and if you try to click on one of the "extra" instance's reference it will crash the preview. So I would say its probably not a good idea at this time to add an object that has multiple instances to the los behaviors "add obstacle". Instead you can place the object you know will have multiple instances into a family and add that family to your "add obstacle" event.
trigger once while true-> los add obstacle family.black
Fixed cap in link.