As a part of SERVER-30761, we refactored parseInExpression() and changed a loop that looked like

BSONObjIterator iter(theArray);
while (iter.more()) {
    auto e = iter.next();
    // ...
}

into

for (auto e : theArray) {
    // ...
}

This has caused a performance regression of roughly 5% when parsing a $in, and is apparent with very large in expressions. The real fix seems like achieving performance parity between the two ways to iterate through a BSONObj, but I'm not sure how much we want to dive down the rabbit hole in terms of seeing the machine code that's generated by the two constructs.