Lag

I decided to start playing my survival world with the latest snapshot. So I installed the new launcher and started playing, when I start my FPS is about 79 running highest settings. Normally its at about 150 FPS. And as I go on my FPS drops to about 9-15 FPS. So I put my setting down to the lowest they can go and then it starts off at a high FPS and then drops to about 20 FPS. So I am running 20 FPS at lowest settings when normally running at 150 FPS highest settings!