First of all, Streams are not meant to achieve these kind of tasks (as opposed to a classic data structure). If you know already how many elements your stream will be processing you might go with the following option, which is, I repeat, not the goal of streams.
Stream<MyData> myStream = readData();
final AtomicInteger loader = new AtomicInteger();
int fivePercent = elementsCount / 20;
MyResult result = myStream
.map(row -> process(row))
.peek(stat -> {
if (loader.incrementAndGet() % fivePercent == 0) {
System.out.println(loader.get() + " elements on " + elementsCount + " treated");
System.out.println((5*(loader.get() / fivePercent)) + "%");
}
})
.reduce(MyStat::aggregate);