diff --git a/pkg/core/region_test.go b/pkg/core/region_test.go
index 804bc052cc8b..8956bd8a357e 100644
--- a/pkg/core/region_test.go
+++ b/pkg/core/region_test.go
@@ -642,30 +642,50 @@ func BenchmarkUpdateBuckets(b *testing.B) {
 }
 
 func BenchmarkRandomRegion(b *testing.B) {
-	regions := NewRegionsInfo()
-	for i := 0; i < 5000000; i++ {
-		peer := &metapb.Peer{StoreId: 1, Id: uint64(i + 1)}
-		region := NewRegionInfo(&metapb.Region{
-			Id:       uint64(i + 1),
-			Peers:    []*metapb.Peer{peer},
-			StartKey: []byte(fmt.Sprintf("%20d", i)),
-			EndKey:   []byte(fmt.Sprintf("%20d", i+1)),
-		}, peer)
-		origin, overlaps, rangeChanged := regions.SetRegion(region)
-		regions.UpdateSubTree(region, origin, overlaps, rangeChanged)
-	}
-	b.Run("random region", func(b *testing.B) {
-		b.ResetTimer()
-		for i := 0; i < b.N; i++ {
-			regions.randLeaderRegion(1, nil)
+	for _, size := range []int{10, 100, 1000, 10000, 100000, 1000000, 10000000} {
+		regions := NewRegionsInfo()
+		for i := 0; i < size; i++ {
+			peer := &metapb.Peer{StoreId: 1, Id: uint64(i + 1)}
+			region := NewRegionInfo(&metapb.Region{
+				Id:       uint64(i + 1),
+				Peers:    []*metapb.Peer{peer},
+				StartKey: []byte(fmt.Sprintf("%20d", i)),
+				EndKey:   []byte(fmt.Sprintf("%20d", i+1)),
+			}, peer)
+			origin, overlaps, rangeChanged := regions.SetRegion(region)
+			regions.UpdateSubTree(region, origin, overlaps, rangeChanged)
 		}
-	})
-	b.Run("random regions", func(b *testing.B) {
-		b.ResetTimer()
-		for i := 0; i < b.N; i++ {
-			regions.RandLeaderRegions(1, nil)
+		b.Run(fmt.Sprintf("random region whole range with size %d", size), func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				regions.randLeaderRegion(1, nil)
+			}
+		})
+		b.Run(fmt.Sprintf("random regions whole range with size %d", size), func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				regions.RandLeaderRegions(1, nil)
+			}
+		})
+		ranges := []KeyRange{
+			NewKeyRange(fmt.Sprintf("%20d", 0), fmt.Sprintf("%20d", size/4)),
+			NewKeyRange(fmt.Sprintf("%20d", size/4), fmt.Sprintf("%20d", size/2)),
+			NewKeyRange(fmt.Sprintf("%20d", size/2), fmt.Sprintf("%20d", size*3/4)),
+			NewKeyRange(fmt.Sprintf("%20d", size*3/4), fmt.Sprintf("%20d", size)),
 		}
-	})
+		b.Run(fmt.Sprintf("random region given ranges with size %d", size), func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				regions.randLeaderRegion(1, ranges)
+			}
+		})
+		b.Run(fmt.Sprintf("random regions given ranges with size %d", size), func(b *testing.B) {
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				regions.RandLeaderRegions(1, ranges)
+			}
+		})
+	}
 }
 
 func BenchmarkRandomSetRegion(b *testing.B) {
diff --git a/pkg/core/region_tree.go b/pkg/core/region_tree.go
index 40e0988b9c3f..9cc19e29c249 100644
--- a/pkg/core/region_tree.go
+++ b/pkg/core/region_tree.go
@@ -341,9 +341,7 @@ func (t *regionTree) RandomRegions(n int, ranges []KeyRange) []*RegionInfo {
 	if t.length() == 0 || n < 1 {
 		return nil
 	}
-	if len(ranges) == 0 {
-		ranges = []KeyRange{NewKeyRange("", "")}
-	}
+	// Pre-allocate the variables to reduce the temporary memory allocations.
 	var (
 		startKey, endKey              []byte
 		startIndex, endIndex, randIdx int
@@ -352,15 +350,10 @@ func (t *regionTree) RandomRegions(n int, ranges []KeyRange) []*RegionInfo {
 		region                        *RegionInfo
 		regions                       = make([]*RegionInfo, 0, n)
 		rangeNum, curLen              = len(ranges), len(regions)
-	)
-	// Keep retrying until we get enough regions.
-	for curLen < n {
-		// Shuffle the ranges to increase the randomness.
-		rand.Shuffle(rangeNum, func(i, j int) {
-			ranges[i], ranges[j] = ranges[j], ranges[i]
-		})
-		for _, r := range ranges {
-			startKey, endKey = r.StartKey, r.EndKey
+		// setStartEndIndices is a helper function to set `startIndex` and `endIndex`
+		// according to the `startKey` and `endKey`.
+		// TODO: maybe we could cache the `startIndex` and `endIndex` for each range.
+		setStartEndIndices = func() {
 			pivotItem.meta.StartKey = startKey
 			startItem, startIndex = t.tree.GetWithIndex(pivotItem)
 			if len(endKey) != 0 {
@@ -374,6 +367,45 @@ func (t *regionTree) RandomRegions(n int, ranges []KeyRange) []*RegionInfo {
 			if startIndex != 0 && startItem == nil && t.tree.GetAt(startIndex-1).Contains(startKey) {
 				startIndex--
 			}
+		}
+	)
+	// If no ranges specified, select randomly from the whole tree.
+	// This is a fast path to reduce the unnecessary iterations.
+	if rangeNum == 0 {
+		startKey, endKey = []byte(""), []byte("")
+		setStartEndIndices()
+		if endIndex <= startIndex {
+			if len(endKey) > 0 && bytes.Compare(startKey, endKey) > 0 {
+				log.Error("wrong range keys",
+					logutil.ZapRedactString("start-key", string(HexRegionKey(startKey))),
+					logutil.ZapRedactString("end-key", string(HexRegionKey(endKey))),
+					errs.ZapError(errs.ErrWrongRangeKeys))
+			}
+			return regions
+		}
+		for curLen < n {
+			randIdx = rand.Intn(endIndex-startIndex) + startIndex
+			region = t.tree.GetAt(randIdx).RegionInfo
+			if curLen < n && region.isInvolved(startKey, endKey) {
+				regions = append(regions, region)
+				curLen++
+			} else if curLen == n {
+				break
+			}
+			// No region found, directly break to avoid infinite loop.
+			if curLen == 0 {
+				break
+			}
+		}
+		return regions
+	}
+	// When there are multiple ranges provided,
+	// keep retrying until we get enough regions.
+	for curLen < n {
+		// Shuffle the ranges to increase the randomness.
+		for _, i := range rand.Perm(rangeNum) {
+			startKey, endKey = ranges[i].StartKey, ranges[i].EndKey
+			setStartEndIndices()
 			if endIndex <= startIndex {
 				if len(endKey) > 0 && bytes.Compare(startKey, endKey) > 0 {
 					log.Error("wrong range keys",
@@ -393,9 +425,8 @@ func (t *regionTree) RandomRegions(n int, ranges []KeyRange) []*RegionInfo {
 				return regions
 			}
 		}
-		// No region found in the given ranges, directly break.
-		// This is to avoid infinite loop.
-		if len(regions) == 0 {
+		// No region found, directly break to avoid infinite loop.
+		if curLen == 0 {
 			break
 		}
 	}