How many objects are you talking about here? The size of a level itself has nothing to do with how fast it runs, it's just essentially numbers that control scrolling - so it's entirely the object count and the processing the game does. To see if it is the LOS, try (backing up your game then) deleting the LOS related stuff and seeing if it runs much faster. If it doesn't, look elsewhere.
Objects offscreen are not drawn - and that's about it. They'll still be processed by events, check collisions, etc. Pathfinding with the RTS movement can be a CPU hog too if used wrong, are you using that?
Also, if you're using shaders, which ones are they? A lot of shaders can slow down performance, especially on lower end cards.