fastest way to find the intersection of n lists of sets

[Prateek]

I have 3 variable length lists of sets. I need to find the common
elements in each list (across sets) really really quickly.

Here is some sample code:

# Doesn't make sense to union the sets - we're going to do
intersections later anyway
l1 = reduce(operator.add, list(x) for x in l1)
l2 = reduce(operator.add, list(x) for x in l2)
l3 = reduce(operator.add, list(x) for x in l3)

# Should I do this in two steps? Maybe by intersecting the two
shortest lists first?
s = frozenset(l1) & frozenset(l2) & frozenset(l3)

I'm assuming frozensets are (somehow) quicker than sets because
they're immutable.

Any code suggestions? Maybe using something in the new fancy-schmancy
itertools module?

Thanks,
Prateek