In the two latest snapshots, there is a constant unplayable lag issue.

Chunks are taking a long time to draw on the screen when game is first entered. Sitting "paused" at the menu screen causes chunks to load marginally faster, but when "Chunk Updates" drops to 0 and stays there, and you return to game, the game immediately starts updating large quantities of chunks (80-100+ at times).

Gameplay is nearly impossible, as blocks break properly and particles are shown, but the block remains on the screen for 5 sec to 2 min. It is possible to walk into these "invisible not-blocks" and have an x-ray effect. Hitboxes for blocks behave normally, it just takes an extremely long time for the screen to reflect any changes.

I experienced this with a fresh new world in 14w29b twice. Opening the world in 1.7.10 or earlier, including earlier snapshots, cures this and normal gameplay resumes.