I have same issue too. Still concern in 1.8pre-1. When I stop and dont move the camera I get back up to 60fps (vsync on). Whenever I move the camera or move away from my last position or whatever makes the new chunks load drops my fps to 10-20. Sometimes even 0 and game stops.
Note: I tested this on different graphics options but it affects every single option and every single render distance.
Note 2: I have Intel Core i5 M430 and my cpu usage goes up to %100 whenever I move.
Maybe it only affects low-mid range GPUs. Because HD 5650 is above optimal but it is not a choppy card.
Still in pre3. Tested on 12, 10 and 8 render distances. All of them are the same. This just doesnt give less fps. This makes the game unplayable!