diff --git a/lib/uint64set/uint64set.go b/lib/uint64set/uint64set.go
index ad826a244..f9d6209fb 100644
--- a/lib/uint64set/uint64set.go
+++ b/lib/uint64set/uint64set.go
@@ -45,6 +45,14 @@ func (s *Set) Clone() *Set {
 	return &dst
 }
 
+func (s *Set) fixItemsCount() {
+	n := 0
+	for i := range s.buckets {
+		n += s.buckets[i].getLen()
+	}
+	s.itemsCount = n
+}
+
 func (s *Set) cloneShallow() *Set {
 	var dst Set
 	dst.itemsCount = s.itemsCount
@@ -177,27 +185,54 @@ func (s *Set) UnionMayOwn(a *Set) {
 }
 
 func (s *Set) union(a *Set, mayOwn bool) {
-	if mayOwn && s.Len() < a.Len() {
-		// Swap `a` with `s` in order to reduce the number of iterations in ForEach loop below.
-		// This operation is safe only if `a` is no longer used after the call to union.
-		*a, *s = *s, *a
-	}
 	if a.Len() == 0 {
 		// Fast path - nothing to union.
 		return
 	}
 	if s.Len() == 0 {
-		// Fast path - just copy a.
-		aCopy := a.Clone()
-		*s = *aCopy
+		// Fast path - copy `a` to `s`.
+		if !mayOwn {
+			a = a.Clone()
+		}
+		*s = *a
 		return
 	}
-	a.ForEach(func(part []uint64) bool {
-		for _, x := range part {
-			s.Add(x)
+	// Make shallow copy of `a`, since it can be modified by a.sort().
+	if !mayOwn {
+		a = a.cloneShallow()
+	}
+	a.sort()
+	s.sort()
+	i := 0
+	j := 0
+	sbuckets := s.buckets
+	for {
+		for i < len(sbuckets) && j < len(a.buckets) && sbuckets[i].hi < a.buckets[j].hi {
+			i++
 		}
-		return true
-	})
+		if i >= len(sbuckets) {
+			for j < len(a.buckets) {
+				b32 := s.addBucket32()
+				a.buckets[j].copyTo(b32)
+				j++
+			}
+			break
+		}
+		for j < len(a.buckets) && a.buckets[j].hi < sbuckets[i].hi {
+			b32 := s.addBucket32()
+			a.buckets[j].copyTo(b32)
+			j++
+		}
+		if j >= len(a.buckets) {
+			break
+		}
+		if sbuckets[i].hi == a.buckets[j].hi {
+			sbuckets[i].union(&a.buckets[j], mayOwn)
+			i++
+			j++
+		}
+	}
+	s.fixItemsCount()
 }
 
 // Intersect removes all the items missing in a from s.
@@ -207,11 +242,10 @@ func (s *Set) Intersect(a *Set) {
 		*s = Set{}
 		return
 	}
-	// Make shallow copy of `a`, since it can be modified below.
+	// Make shallow copy of `a`, since it can be modified by a.sort().
 	a = a.cloneShallow()
 	a.sort()
 	s.sort()
-	itemsCount := 0
 	i := 0
 	j := 0
 	for {
@@ -233,12 +267,12 @@ func (s *Set) Intersect(a *Set) {
 			break
 		}
 		if s.buckets[i].hi == a.buckets[j].hi {
-			itemsCount += s.buckets[i].intersect(&a.buckets[j])
+			s.buckets[i].intersect(&a.buckets[j])
 			i++
 			j++
 		}
 	}
-	s.itemsCount = itemsCount
+	s.fixItemsCount()
 }
 
 // Subtract removes from s all the shared items between s and a.
@@ -307,11 +341,56 @@ func (b *bucket32) cloneShallow() *bucket32 {
 	return &dst
 }
 
-func (b *bucket32) intersect(a *bucket32) int {
-	a = a.cloneShallow() // clone a, since is is sorted below.
+func (b *bucket32) getLen() int {
+	n := 0
+	for i := range b.buckets {
+		n += b.buckets[i].getLen()
+	}
+	return n
+}
+
+func (b *bucket32) union(a *bucket32, mayOwn bool) {
+	if !mayOwn {
+		a = a.cloneShallow() // clone a, since it is sorted below.
+	}
+	a.sort()
+	b.sort()
+	i := 0
+	j := 0
+	bb16his := b.b16his
+	for {
+		for i < len(bb16his) && j < len(a.b16his) && bb16his[i] < a.b16his[j] {
+			i++
+		}
+		if i >= len(bb16his) {
+			for j < len(a.b16his) {
+				b16 := b.addBucket16()
+				a.buckets[j].copyTo(b16)
+				j++
+			}
+			break
+		}
+		for j < len(a.b16his) && a.b16his[j] < bb16his[i] {
+			b16 := b.addBucket16()
+			a.buckets[j].copyTo(b16)
+			j++
+		}
+		if j >= len(a.b16his) {
+			break
+		}
+		if bb16his[i] == a.b16his[j] {
+			b.buckets[i].union(&a.buckets[j])
+			i++
+			j++
+		}
+	}
+	b.sort()
+}
+
+func (b *bucket32) intersect(a *bucket32) {
+	a = a.cloneShallow() // clone a, since it is sorted below.
 	a.sort()
 	b.sort()
-	itemsCount := 0
 	i := 0
 	j := 0
 	for {
@@ -333,12 +412,11 @@ func (b *bucket32) intersect(a *bucket32) int {
 			break
 		}
 		if b.b16his[i] == a.b16his[j] {
-			itemsCount += b.buckets[i].intersect(&a.buckets[j])
+			b.buckets[i].intersect(&a.buckets[j])
 			i++
 			j++
 		}
 	}
-	return itemsCount
 }
 
 func (b *bucket32) forEach(f func(part []uint64) bool) bool {
@@ -541,36 +619,65 @@ type bucket16 struct {
 	smallPool    [56]uint16
 }
 
-func (b *bucket16) intersect(a *bucket16) int {
-	itemsCount := 0
+func (b *bucket16) getLen() int {
+	if b.bits == nil {
+		return b.smallPoolLen
+	}
+	n := 0
+	for _, x := range b.bits {
+		if x > 0 {
+			n += bits.OnesCount64(x)
+		}
+	}
+	return n
+}
+
+func (b *bucket16) union(a *bucket16) {
+	if a.bits != nil && b.bits != nil {
+		// Fast path - use bitwise ops.
+		for i, ax := range a.bits {
+			bx := b.bits[i]
+			bx |= ax
+			b.bits[i] = bx
+		}
+		return
+	}
+
+	// Slow path
+	xbuf := partBufPool.Get().(*[]uint64)
+	buf := *xbuf
+	buf = a.appendTo(buf[:0], 0, 0)
+	for _, x := range buf {
+		x16 := uint16(x)
+		b.add(x16)
+	}
+	*xbuf = buf
+	partBufPool.Put(xbuf)
+}
+
+func (b *bucket16) intersect(a *bucket16) {
 	if a.bits != nil && b.bits != nil {
 		// Fast path - use bitwise ops
 		for i, ax := range a.bits {
 			bx := b.bits[i]
 			bx &= ax
-			if bx > 0 {
-				itemsCount += bits.OnesCount64(bx)
-			}
 			b.bits[i] = bx
 		}
-		return itemsCount
+		return
 	}
 
 	// Slow path
 	xbuf := partBufPool.Get().(*[]uint64)
 	buf := *xbuf
 	buf = b.appendTo(buf[:0], 0, 0)
-	itemsCount = len(buf)
 	for _, x := range buf {
 		x16 := uint16(x)
 		if !a.has(x16) {
 			b.del(x16)
-			itemsCount--
 		}
 	}
 	*xbuf = buf
 	partBufPool.Put(xbuf)
-	return itemsCount
 }
 
 func (b *bucket16) sizeBytes() uint64 {