I just found, that nether and the_end tend to lag.
And so I began investigating.
Here's what I found:
Smooth Lighting setting does not work in the worlds nether and the_end.
Let's say I have turned Smooth Lighting off, I enter the nether, and regardless of my setting, Smooth Lighting always stays ON.
//14w30c
Nether is safe from this bug.
The_end still seems to be affected by this bug.
I could replicate the issue in the Nether (for both 1.7.4 and 14w07a), but not the end.
Interestingly, if you START your game in the Nether with smooth lighting off, it seems to be ok.