Lukas Mayrhofer
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I've investigated this issue a bit and was able to identify the culprit as the complex "mobSpawner" procedure.
The corresponding method is obfuscated as ara/a(qs, boolean, boolean, boolean) in the snapshot 14w33a.
In a superflat world, during a single invoke of that method, around 7000 instances of entities are created using reflection in the inner-most loop... per tick.
Inside this inner loop, after an Entity was created, a verification process follows, checking if that particular Entity is suitable for a randomly generated position.
If this verification process fails, the entity is dismissed and the inner-most loop continues to run far too "long".
I slightly modified this method by inserting an additional control-flow statement after a verification failure that jumps into one of the outer-loops that iterates the valid chunks.
This modification may not follow intended behavior, but significantly reduced the time spent on this method and stabilized the tick-rate back to normal.
13w47d didn't fix this issue.
Can confirm.
Please reopen this issue. It wasn't fixed by 13w42b and seems to be a OS X-specific problem.
Confirmed in 14w34c.
The mob spawner creates and discards thousands of entities per tick in superflat worlds, causing the reduced tick-speed.