See: http://richardstartin.uk/spliterator-characteristics-and-performance/

    @Benchmark
    public long countRange() {
        return IntStream.range(0, size).count();
    }

    @Benchmark
    public long countRangeDistinct() {
        return IntStream.range(0, size).distinct().count();
    }

Currently countRange is always quick (15ns/op) independent of size, but countRange scales with size:

Benchmark (size) Mode Cnt Score Error Units
StreamBench.countRange 1024 avgt 5 17.160 ± 8.957 ns/op
StreamBench.countRangeDistinct 1024 avgt 5 34025.817 ± 15145.915 ns/op

Benchmark (size) Mode Cnt Score Error Units
StreamBench.countRange 264444 avgt 5 15.952 ± 5.163 ns/op
StreamBench.countRangeDistinct 264444 avgt 5 2191633.584 ± 2738022.121 ns/op

With a patch[1] to trust Streams that are already distinct, the latter op is optimized for all size inputs:

Benchmark (size) Mode Cnt Score Error Units
StreamBench.countRange 264444 avgt 5 15.439 ± 1.842 ns/op
StreamBench.countRangeDistinct 264444 avgt 5 32.939 ± 4.268 ns/op

[1]
diff -r 0ee20aad71c4 src/java.base/share/classes/java/util/stream/AbstractPipeline.java
--- a/src/java.base/share/classes/java/util/stream/AbstractPipeline.java Thu Dec 14 16:05:08 2017 +0100
+++ b/src/java.base/share/classes/java/util/stream/AbstractPipeline.java Fri Dec 15 20:27:27 2017 +0100
@@ -509,6 +509,10 @@
         return combinedFlags;
     }
 
+ final boolean isDistinct() {
+ return StreamOpFlag.DISTINCT.isKnown(combinedFlags);
+ }
+
     final boolean isOrdered() {
         return StreamOpFlag.ORDERED.isKnown(combinedFlags);
     }
diff -r 0ee20aad71c4 src/java.base/share/classes/java/util/stream/ReferencePipeline.java
--- a/src/java.base/share/classes/java/util/stream/ReferencePipeline.java Thu Dec 14 16:05:08 2017 +0100
+++ b/src/java.base/share/classes/java/util/stream/ReferencePipeline.java Fri Dec 15 20:27:27 2017 +0100
@@ -383,6 +383,9 @@
 
     @Override
     public final Stream<P_OUT> distinct() {
+ if (this.isDistinct()) {
+ return this;
+ }
         return DistinctOps.makeRef(this);
     }