Did not affect 0.14.1/0.14.0.
The framerate drops seemingly random while being in caves or looking at huge, sponge-like structures.
I was able to track down the cause to too many/any entities just "being there" somewhere in the world within the player's view angle. The entities do not need to be visible, it is sufficient for them to be in the possible view angle.
I suspect some kind of rendering optimization algorithm not taking entities into account.
Why entities? I commented out the code in "/data/shaders/entity.vertex". After restarting the game, the random lags were completely gone.

What I expected to happen:
No framerate drop with entities existing behind blocks but not being visible to player.

What actually happened:
Heavy, unexpected framerate drop with entities behind blocks etc.

Steps to reproduce:
1) Find a monster spawner in a cave.
2) Wait for enough monsters to spawn.
3) Walk around in the cave. You will notice a framerate drop.