Yep, as I said, what was really making the game laggy was the blur on lightning and the motion blur.
Deactivating one of the two would give me some extra FPS and deactivating both would give me back my 60FPS, so I think that's what I'll do .
Thank you anyway!
(Btw, I found a way to put two events in the same box even if they shouldn't be. Like the "Trigger once while true" and "On function" and so on. For the moment, it helps me doing some things and don't seem to be creating any bug or glitch, so I would rather say it's a tip and trick, but, if it happened that it may cause any problem, here is how I did it:
Anyway, that's still useful if a function is called in multiple following frames, like on a "Is key pressed" condition: