diff --git a/dag/set.go b/dag/set.go
index fc16e801b..2e5d85b2d 100644
--- a/dag/set.go
+++ b/dag/set.go
@@ -38,17 +38,18 @@ func (s Set) Include(v interface{}) bool {
 // Intersection computes the set intersection with other.
 func (s Set) Intersection(other Set) Set {
 	result := make(Set)
-	if s == nil {
+	if s == nil || other == nil {
 		return result
 	}
-	if other != nil {
-		for _, v := range s {
-			if other.Include(v) {
-				result.Add(v)
-			}
+	// Iteration over a smaller set has better performance.
+	if other.Len() < s.Len() {
+		s, other = other, s
+	}
+	for _, v := range s {
+		if other.Include(v) {
+			result.Add(v)
 		}
 	}
-
 	return result
 }
 
diff --git a/dag/set_test.go b/dag/set_test.go
index 36bd6a65b..d59eacfbd 100644
--- a/dag/set_test.go
+++ b/dag/set_test.go
@@ -119,3 +119,31 @@ func TestSetCopy(t *testing.T) {
 	}
 
 }
+
+func makeSet(n int) Set {
+	ret := make(Set, n)
+	for i := 0; i < n; i++ {
+		ret.Add(i)
+	}
+	return ret
+}
+
+func BenchmarkSetIntersection_100_100000(b *testing.B) {
+	small := makeSet(100)
+	large := makeSet(100000)
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		small.Intersection(large)
+	}
+}
+
+func BenchmarkSetIntersection_100000_100(b *testing.B) {
+	small := makeSet(100)
+	large := makeSet(100000)
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		large.Intersection(small)
+	}
+}