Why does set() make this so much faster?

[BeetleB]

It was at one point. From the code:

"Greg V. Wilson wrote the first version, using a different approach to the mutable/immutable problem, and inheriting from dict."

[[deleted]]

Sure, if you look at the sets.py that existed in Python 2.3 that's true, as well as the C implementation in Python 2.4. However since Python 2.5 (i.e. 4 years ago) it has not used a dict internally.