I decided to use iterators to work with collections in a functional style at some point in development, but immediately noticed a CPU time performance drawdown. So I decided to do a simple performance test of different iterating approaches by getting each element and adding it into a new list (to eliminate possible dead code):

List<String> strings = new List<String>();
for (Integer i = 0; i < 50000; i++) {
    strings.add('');
}
Iterator<String> iterator = strings.iterator();
List<String> strs = new List<String>();

// Start measuring
Integer start = Limits.getCpuTime();
/*
// optimized indexed for loop
for (Integer i = 0, size = strings.size(); i < size; i++) strs.add(strings[i]);
// indexed for loop
for (Integer i = 0; i < strings.size(); i++) strs.add(strings[i]);
// indexed get for loop
for (Integer i = 0; i < strings.size(); i++) strs.add(strings.get(i));
// for each loop
for (String s : strings) strs.add(s);
// iterator for loop
for (;iterator.hasNext();) strs.add(iterator.next());
// iterator while loop
while (iterator.hasNext()) strs.add(iterator.next());
*/
// End measuring
Integer duration = (Limits.getCpuTime() - start);
System.debug(LoggingLevel.INFO, duration);

Results for 50 trials each of 50.000 iterations:

I am aware that results depend on many factors and may vary, and I also know that adding an element to a list also consumes some CPU time. But the relative results speak for themselves. I've also tested the same loops in JAVA but the results did not differ as much from each other as in APEX. It turns out that iterating through collections using an iterator is several times slower than using for loops, which negates all its usefulness in similar scenarios.

Questions:

1. Why is iterating through collections with an iterator SO slow or what I'm missing?
2. Is there a way to speed up or optimize iterators somehow when it comes to getting every single element of collection? (Custom / [Lazy] iterators are even slower)