Why is Minecraft running so slow all of a sudden?

Maybe because loads of people are getting on it so the whole system is laggy. PewDiePie started playing so I think it has sparked a huge interest for it in the past week

Try to look at memory usage in game by pressing F3, right corner if I remember correctly.

So if your game reached the memory limit that will make suddenly slow, maybe the setting game was changed by reason.

Well if it wasn't and memory limit is still in normal or not too low. Sorry I didn't know more about that