It looks like you get really bad performance when you are at the ground of oceans or large areas of water. This apparently started happening in 17w47b, 17w47a is not affected.
This makes my FPS drop down to 2 while in previous versions I had about 60. Interestingly when pausing the game I get back to about 60 FPS.
- Create a superflat world with the preset "Water World"
- Go to the ground