See demonstration of the issue.

The standard library method Iterator#++ creates a JoinIterator. The hasNext method on JoinIterator always calls the left iterator's hasNext method, rather than remembering that it has been exhausted.

In cases where the left iterator's hasNext method is expensive, such as any kind of IO, this was at the root of a baffling performance problem.

This behavior is fixed in ConcatIterator, which never calls the underlying hasNext a second time for an iterator which already returned false. So that provides an easy, if dirty, workaround:

val it1 = new IteratorWithExpensiveHasNext()
val it2 = new IteratorWithExpensiveHasNext()

// SLOW due to calling it1.hasNext every time
(it1 ++ it2).foreach(_ => ())

// FASTER due to NOT calling it1.hasNext every time
(Iterator.empty ++ it1 ++ it2).foreach(_ => ())

NOTE: you can use #memoizeExhaustion from my iterata project as a workaround, as well.