I once scripted Farmville, and quickly overtook all my friends who played it. Then somewhere at level 60 or 80 the game became so extremely slow that I gave up. The game could only handle a certain number of clicks per second, and the script would have to run for dozens of hours to gain yet another level.
EDIT: That's because gaining levels required XP, and there was literally 1 XP for each click, regardless of what you did. You could figure out a strategy to get more gold, but I already had way more gold than I could spend. But there was no strategy to get more XP other than "click more". Up to certain level it was manageable, I just let the script run when I was away from the computer, and it took a few minutes to gain another level. Up to certain point, the required XP per level grew quadratically; behind that point it started growing exponentially, so I knew that I could never make more than 5 or 10 more levels past that point.
EDIT: That's because gaining levels required XP, and there was literally 1 XP for each click, regardless of what you did. You could figure out a strategy to get more gold, but I already had way more gold than I could spend. But there was no strategy to get more XP other than "click more". Up to certain level it was manageable, I just let the script run when I was away from the computer, and it took a few minutes to gain another level. Up to certain point, the required XP per level grew quadratically; behind that point it started growing exponentially, so I knew that I could never make more than 5 or 10 more levels past that point.