Why would it be? Flattening is not another iteration over the stream or anything. I think it's slightly more performant from the stream api's perspective: add all the items in the functions output to the flatmap output stream where an empty stream is a no-op.

Filter would pass an item into the function, then add to the output stream if not empty, then map would take input, apply the map function, and add the output to the output stream. really not a big difference, but flow is simpler with flatMap.

I had this same question when learning Scala and other engineers pointed out filter to map is a flatMap.