Why is my minecraft game so laggy?

I'm trying to play minecraft but it's so laggy. It works perfectly as I'm starting the game and on the home menu. As soon as my world loads, I move around for like 10 seconds and it will freeze. It will be stuck for like 10 seconds, and then I run around again and it freezes again for the same time. It will keep doing this.
It's been like this for awhile now. It was perfectly fine before but now it's being stupid. I can't even play so I just quit out of frustration.
Is it the newest update or is it just my game? Do I have to delete it and reinstall it?

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