TreeMap sub-maps use the default IteratorSpliterator implementation for EntrySetView which is slow for some operations, because EntrySetView.size() iterates all elements. This is most trivially shown by something like `largeTreeMap.tailMap(0L, false).entrySet().limit(1).count()` taking a long time.

keySet() does not have the same problem, as it provides a custom Spliterator implementation which is not Spliterator.SIZED, and returns Long.MAX_VALUE for estimateSize() (which is the recommended approach when the size is expensive to compute).

```
import java.util.Collection;
import java.util.TreeMap;

void main() {
    var map = new TreeMap<Long, String>();
    for (long i = 0; i < 10_000_000; i++) map.put(i, Long.toString(i));
    test(map.tailMap(0L, false).keySet());
    test(map.tailMap(0L, false).entrySet());
}

void test(Collection x) {
    IO.println(x.getClass());
    long start = System.nanoTime();
    x.stream().limit(1).count();
    IO.println("\t.stream().limit(1).count() took " + (System.nanoTime() - start) / 1_000_000.0 + "ms");
    IO.println("\tspliterator = " + x.spliterator().getClass().getSimpleName() + ", estimateSize() = " + x.spliterator().estimateSize());
}
```

```
class java.util.TreeMap$KeySet
    .stream().limit(1).count() took 0.421209ms
    spliterator = SubMapKeyIterator, estimateSize() = 9223372036854775807
class java.util.TreeMap$AscendingSubMap$AscendingEntrySetView
    .stream().limit(1).count() took 141.4435ms
    spliterator = IteratorSpliterator, estimateSize() = 9999999
```