diff --git a/cartographer/BUILD.bazel b/cartographer/BUILD.bazel
index 063489a..0caf39a 100644
--- a/cartographer/BUILD.bazel
+++ b/cartographer/BUILD.bazel
@@ -98,6 +98,7 @@ cc_library(
         "@com_google_absl//absl/base",
         "@com_google_absl//absl/strings",
         "@com_google_absl//absl/container:flat_hash_map",
+        "@com_google_absl//absl/container:flat_hash_set",
         "@com_google_absl//absl/synchronization",
         "@com_google_absl//absl/types:optional",
         "@com_google_glog//:glog",
diff --git a/cartographer/io/fixed_ratio_sampling_points_processor.cc b/cartographer/io/fixed_ratio_sampling_points_processor.cc
index 8ac14de..6b07f3c 100644
--- a/cartographer/io/fixed_ratio_sampling_points_processor.cc
+++ b/cartographer/io/fixed_ratio_sampling_points_processor.cc
@@ -42,7 +42,7 @@ FixedRatioSamplingPointsProcessor::FixedRatioSamplingPointsProcessor(
 
 void FixedRatioSamplingPointsProcessor::Process(
     std::unique_ptr<PointsBatch> batch) {
-  std::unordered_set<int> to_remove;
+  absl::flat_hash_set<int> to_remove;
   for (size_t i = 0; i < batch->points.size(); ++i) {
     if (!sampler_->Pulse()) {
       to_remove.insert(i);
diff --git a/cartographer/io/frame_id_filtering_points_processor.cc b/cartographer/io/frame_id_filtering_points_processor.cc
index f9b9f42..5fbe4c4 100644
--- a/cartographer/io/frame_id_filtering_points_processor.cc
+++ b/cartographer/io/frame_id_filtering_points_processor.cc
@@ -37,14 +37,14 @@ FrameIdFilteringPointsProcessor::FromDictionary(
         dictionary->GetDictionary("drop_frames")->GetArrayValuesAsStrings();
   }
   return absl::make_unique<FrameIdFilteringPointsProcessor>(
-      std::unordered_set<std::string>(keep_frames.begin(), keep_frames.end()),
-      std::unordered_set<std::string>(drop_frames.begin(), drop_frames.end()),
+      absl::flat_hash_set<std::string>(keep_frames.begin(), keep_frames.end()),
+      absl::flat_hash_set<std::string>(drop_frames.begin(), drop_frames.end()),
       next);
 }
 
 FrameIdFilteringPointsProcessor::FrameIdFilteringPointsProcessor(
-    const std::unordered_set<std::string>& keep_frame_ids,
-    const std::unordered_set<std::string>& drop_frame_ids,
+    const absl::flat_hash_set<std::string>& keep_frame_ids,
+    const absl::flat_hash_set<std::string>& drop_frame_ids,
     PointsProcessor* next)
     : keep_frame_ids_(keep_frame_ids),
       drop_frame_ids_(drop_frame_ids),
diff --git a/cartographer/io/frame_id_filtering_points_processor.h b/cartographer/io/frame_id_filtering_points_processor.h
index 598b832..a35cb60 100644
--- a/cartographer/io/frame_id_filtering_points_processor.h
+++ b/cartographer/io/frame_id_filtering_points_processor.h
@@ -17,8 +17,7 @@
 #ifndef CARTOGRAPHER_IO_FRAME_ID_FILTERING_POINTS_PROCESSOR_H_
 #define CARTOGRAPHER_IO_FRAME_ID_FILTERING_POINTS_PROCESSOR_H_
 
-#include <unordered_set>
-
+#include "absl/container/flat_hash_set.h"
 #include "cartographer/common/lua_parameter_dictionary.h"
 #include "cartographer/io/points_processor.h"
 
@@ -32,8 +31,8 @@ class FrameIdFilteringPointsProcessor : public PointsProcessor {
  public:
   constexpr static const char* kConfigurationFileActionName = "frame_id_filter";
   FrameIdFilteringPointsProcessor(
-      const std::unordered_set<std::string>& keep_frame_ids,
-      const std::unordered_set<std::string>& drop_frame_ids,
+      const absl::flat_hash_set<std::string>& keep_frame_ids,
+      const absl::flat_hash_set<std::string>& drop_frame_ids,
       PointsProcessor* next);
   static std::unique_ptr<FrameIdFilteringPointsProcessor> FromDictionary(
       common::LuaParameterDictionary* dictionary, PointsProcessor* next);
@@ -48,8 +47,8 @@ class FrameIdFilteringPointsProcessor : public PointsProcessor {
   FlushResult Flush() override;
 
  private:
-  const std::unordered_set<std::string> keep_frame_ids_;
-  const std::unordered_set<std::string> drop_frame_ids_;
+  const absl::flat_hash_set<std::string> keep_frame_ids_;
+  const absl::flat_hash_set<std::string> drop_frame_ids_;
   PointsProcessor* const next_;
 };
 
diff --git a/cartographer/io/min_max_range_filtering_points_processor.cc b/cartographer/io/min_max_range_filtering_points_processor.cc
index 898a82c..c487d6d 100644
--- a/cartographer/io/min_max_range_filtering_points_processor.cc
+++ b/cartographer/io/min_max_range_filtering_points_processor.cc
@@ -38,7 +38,7 @@ MinMaxRangeFiteringPointsProcessor::MinMaxRangeFiteringPointsProcessor(
 
 void MinMaxRangeFiteringPointsProcessor::Process(
     std::unique_ptr<PointsBatch> batch) {
-  std::unordered_set<int> to_remove;
+  absl::flat_hash_set<int> to_remove;
   for (size_t i = 0; i < batch->points.size(); ++i) {
     const float range = (batch->points[i].position - batch->origin).norm();
     if (!(min_range_ <= range && range <= max_range_)) {
diff --git a/cartographer/io/outlier_removing_points_processor.cc b/cartographer/io/outlier_removing_points_processor.cc
index 5cff194..92c9787 100644
--- a/cartographer/io/outlier_removing_points_processor.cc
+++ b/cartographer/io/outlier_removing_points_processor.cc
@@ -107,7 +107,7 @@ void OutlierRemovingPointsProcessor::ProcessInPhaseTwo(
 void OutlierRemovingPointsProcessor::ProcessInPhaseThree(
     std::unique_ptr<PointsBatch> batch) {
   constexpr double kMissPerHitLimit = 3;
-  std::unordered_set<int> to_remove;
+  absl::flat_hash_set<int> to_remove;
   for (size_t i = 0; i < batch->points.size(); ++i) {
     const VoxelData voxel =
         voxels_.value(voxels_.GetCellIndex(batch->points[i].position));
diff --git a/cartographer/io/pbstream_main.cc b/cartographer/io/pbstream_main.cc
index 6055b77..0a795ff 100644
--- a/cartographer/io/pbstream_main.cc
+++ b/cartographer/io/pbstream_main.cc
@@ -15,8 +15,8 @@
  */
 
 #include <functional>
-#include <unordered_set>
 
+#include "absl/container/flat_hash_set.h"
 #include "cartographer/io/internal/pbstream_info.h"
 #include "cartographer/io/internal/pbstream_migrate.h"
 #include "gflags/gflags.h"
diff --git a/cartographer/io/points_batch.cc b/cartographer/io/points_batch.cc
index 2639cd0..ea0fe4c 100644
--- a/cartographer/io/points_batch.cc
+++ b/cartographer/io/points_batch.cc
@@ -19,7 +19,7 @@
 namespace cartographer {
 namespace io {
 
-void RemovePoints(std::unordered_set<int> to_remove, PointsBatch* batch) {
+void RemovePoints(absl::flat_hash_set<int> to_remove, PointsBatch* batch) {
   const int new_num_points = batch->points.size() - to_remove.size();
   sensor::PointCloud points;
   points.reserve(new_num_points);
diff --git a/cartographer/io/points_batch.h b/cartographer/io/points_batch.h
index 980fec7..752966e 100644
--- a/cartographer/io/points_batch.h
+++ b/cartographer/io/points_batch.h
@@ -19,10 +19,10 @@
 
 #include <array>
 #include <cstdint>
-#include <unordered_set>
 #include <vector>
 
 #include "Eigen/Core"
+#include "absl/container/flat_hash_set.h"
 #include "cartographer/common/time.h"
 #include "cartographer/io/color.h"
 #include "cartographer/sensor/point_cloud.h"
@@ -67,7 +67,7 @@ struct PointsBatch {
 };
 
 // Removes the indices in 'to_remove' from 'batch'.
-void RemovePoints(std::unordered_set<int> to_remove, PointsBatch* batch);
+void RemovePoints(absl::flat_hash_set<int> to_remove, PointsBatch* batch);
 
 }  // namespace io
 }  // namespace cartographer
diff --git a/cartographer/mapping/internal/collated_trajectory_builder.cc b/cartographer/mapping/internal/collated_trajectory_builder.cc
index bfb73af..b72aa26 100644
--- a/cartographer/mapping/internal/collated_trajectory_builder.cc
+++ b/cartographer/mapping/internal/collated_trajectory_builder.cc
@@ -39,7 +39,7 @@ CollatedTrajectoryBuilder::CollatedTrajectoryBuilder(
       trajectory_id_(trajectory_id),
       wrapped_trajectory_builder_(std::move(wrapped_trajectory_builder)),
       last_logging_time_(std::chrono::steady_clock::now()) {
-  std::unordered_set<std::string> expected_sensor_id_strings;
+  absl::flat_hash_set<std::string> expected_sensor_id_strings;
   for (const auto& sensor_id : expected_sensor_ids) {
     if (sensor_id.type == SensorId::SensorType::LANDMARK &&
         !collate_landmarks_) {
diff --git a/cartographer/mapping/internal/connected_components.cc b/cartographer/mapping/internal/connected_components.cc
index 6306b0b..131fb4f 100644
--- a/cartographer/mapping/internal/connected_components.cc
+++ b/cartographer/mapping/internal/connected_components.cc
@@ -17,8 +17,8 @@
 #include "cartographer/mapping/internal/connected_components.h"
 
 #include <algorithm>
-#include <unordered_set>
 
+#include "absl/container/flat_hash_set.h"
 #include "cartographer/mapping/proto/connected_components.pb.h"
 #include "glog/logging.h"
 
diff --git a/cartographer/sensor/collator_interface.h b/cartographer/sensor/collator_interface.h
index 6fff62d..91d7c83 100644
--- a/cartographer/sensor/collator_interface.h
+++ b/cartographer/sensor/collator_interface.h
@@ -19,9 +19,9 @@
 
 #include <functional>
 #include <memory>
-#include <unordered_set>
 #include <vector>
 
+#include "absl/container/flat_hash_set.h"
 #include "absl/types/optional.h"
 #include "cartographer/sensor/data.h"
 
@@ -42,7 +42,7 @@ class CollatorInterface {
   // for each collated sensor data.
   virtual void AddTrajectory(
       int trajectory_id,
-      const std::unordered_set<std::string>& expected_sensor_ids,
+      const absl::flat_hash_set<std::string>& expected_sensor_ids,
       const Callback& callback) = 0;
 
   // Marks 'trajectory_id' as finished.
diff --git a/cartographer/sensor/internal/collator.cc b/cartographer/sensor/internal/collator.cc
index 7db3aa5..917d6be 100644
--- a/cartographer/sensor/internal/collator.cc
+++ b/cartographer/sensor/internal/collator.cc
@@ -21,7 +21,7 @@ namespace sensor {
 
 void Collator::AddTrajectory(
     const int trajectory_id,
-    const std::unordered_set<std::string>& expected_sensor_ids,
+    const absl::flat_hash_set<std::string>& expected_sensor_ids,
     const Callback& callback) {
   for (const auto& sensor_id : expected_sensor_ids) {
     const auto queue_key = QueueKey{trajectory_id, sensor_id};
diff --git a/cartographer/sensor/internal/collator.h b/cartographer/sensor/internal/collator.h
index 053b27e..bea93af 100644
--- a/cartographer/sensor/internal/collator.h
+++ b/cartographer/sensor/internal/collator.h
@@ -19,10 +19,10 @@
 
 #include <functional>
 #include <memory>
-#include <unordered_set>
 #include <vector>
 
 #include "absl/container/flat_hash_map.h"
+#include "absl/container/flat_hash_set.h"
 #include "cartographer/sensor/collator_interface.h"
 #include "cartographer/sensor/data.h"
 #include "cartographer/sensor/internal/ordered_multi_queue.h"
@@ -37,9 +37,10 @@ class Collator : public CollatorInterface {
   Collator(const Collator&) = delete;
   Collator& operator=(const Collator&) = delete;
 
-  void AddTrajectory(int trajectory_id,
-                     const std::unordered_set<std::string>& expected_sensor_ids,
-                     const Callback& callback) override;
+  void AddTrajectory(
+      int trajectory_id,
+      const absl::flat_hash_set<std::string>& expected_sensor_ids,
+      const Callback& callback) override;
 
   void FinishTrajectory(int trajectory_id) override;
 
diff --git a/cartographer/sensor/internal/collator_test.cc b/cartographer/sensor/internal/collator_test.cc
index e0f79b1..6cf9099 100644
--- a/cartographer/sensor/internal/collator_test.cc
+++ b/cartographer/sensor/internal/collator_test.cc
@@ -67,7 +67,7 @@ TEST(Collator, Ordering) {
   Collator collator;
   collator.AddTrajectory(
       kTrajectoryId,
-      std::unordered_set<std::string>(kSensorId.begin(), kSensorId.end()),
+      absl::flat_hash_set<std::string>(kSensorId.begin(), kSensorId.end()),
       [&received, kTrajectoryId](const std::string& sensor_id,
                                  std::unique_ptr<Data> data) {
         received.push_back(CollatorOutput(kTrajectoryId, data->GetSensorId(),
@@ -134,7 +134,7 @@ TEST(Collator, OrderingMultipleTrajectories) {
   Collator collator;
   collator.AddTrajectory(
       kTrajectoryId[0],
-      std::unordered_set<std::string>(kSensorId.begin(), kSensorId.end()),
+      absl::flat_hash_set<std::string>(kSensorId.begin(), kSensorId.end()),
       [&received, kTrajectoryId](const std::string& sensor_id,
                                  std::unique_ptr<Data> data) {
         received.push_back(CollatorOutput(kTrajectoryId[0], data->GetSensorId(),
@@ -142,7 +142,7 @@ TEST(Collator, OrderingMultipleTrajectories) {
       });
   collator.AddTrajectory(
       kTrajectoryId[1],
-      std::unordered_set<std::string>(kSensorId.begin(), kSensorId.end()),
+      absl::flat_hash_set<std::string>(kSensorId.begin(), kSensorId.end()),
       [&received, kTrajectoryId](const std::string& sensor_id,
                                  std::unique_ptr<Data> data) {
         received.push_back(CollatorOutput(kTrajectoryId[1], data->GetSensorId(),
diff --git a/cartographer/sensor/internal/trajectory_collator.cc b/cartographer/sensor/internal/trajectory_collator.cc
index 4b7c89c..982dbbd 100644
--- a/cartographer/sensor/internal/trajectory_collator.cc
+++ b/cartographer/sensor/internal/trajectory_collator.cc
@@ -25,7 +25,7 @@ metrics::Family<metrics::Counter>*
 
 void TrajectoryCollator::AddTrajectory(
     const int trajectory_id,
-    const std::unordered_set<std::string>& expected_sensor_ids,
+    const absl::flat_hash_set<std::string>& expected_sensor_ids,
     const Callback& callback) {
   CHECK_EQ(trajectory_to_queue_.count(trajectory_id), 0);
   for (const auto& sensor_id : expected_sensor_ids) {
diff --git a/cartographer/sensor/internal/trajectory_collator.h b/cartographer/sensor/internal/trajectory_collator.h
index 135a60d..4b7c7e4 100644
--- a/cartographer/sensor/internal/trajectory_collator.h
+++ b/cartographer/sensor/internal/trajectory_collator.h
@@ -42,9 +42,10 @@ class TrajectoryCollator : public CollatorInterface {
   TrajectoryCollator(const TrajectoryCollator&) = delete;
   TrajectoryCollator& operator=(const TrajectoryCollator&) = delete;
 
-  void AddTrajectory(int trajectory_id,
-                     const std::unordered_set<std::string>& expected_sensor_ids,
-                     const Callback& callback) override;
+  void AddTrajectory(
+      int trajectory_id,
+      const absl::flat_hash_set<std::string>& expected_sensor_ids,
+      const Callback& callback) override;
 
   void FinishTrajectory(int trajectory_id) override;
 
diff --git a/cartographer/sensor/internal/trajectory_collator_test.cc b/cartographer/sensor/internal/trajectory_collator_test.cc
index 7f35eb8..cb8f5f5 100644
--- a/cartographer/sensor/internal/trajectory_collator_test.cc
+++ b/cartographer/sensor/internal/trajectory_collator_test.cc
@@ -63,7 +63,7 @@ TEST(TrajectoryCollator, OrderingMultipleTrajectories) {
   TrajectoryCollator collator;
   collator.AddTrajectory(
       kTrajectoryId[0],
-      std::unordered_set<std::string>(kSensorId.begin(), kSensorId.end()),
+      absl::flat_hash_set<std::string>(kSensorId.begin(), kSensorId.end()),
       [&received, kTrajectoryId](const std::string& sensor_id,
                                  std::unique_ptr<Data> data) {
         received.push_back(CollatorOutput(kTrajectoryId[0], data->GetSensorId(),
@@ -71,7 +71,7 @@ TEST(TrajectoryCollator, OrderingMultipleTrajectories) {
       });
   collator.AddTrajectory(
       kTrajectoryId[1],
-      std::unordered_set<std::string>(kSensorId.begin(), kSensorId.end()),
+      absl::flat_hash_set<std::string>(kSensorId.begin(), kSensorId.end()),
       [&received, kTrajectoryId](const std::string& sensor_id,
                                  std::unique_ptr<Data> data) {
         received.push_back(CollatorOutput(kTrajectoryId[1], data->GetSensorId(),
diff --git a/cartographer/sensor/internal/voxel_filter.h b/cartographer/sensor/internal/voxel_filter.h
index a8edbbe..f14575b 100644
--- a/cartographer/sensor/internal/voxel_filter.h
+++ b/cartographer/sensor/internal/voxel_filter.h
@@ -18,8 +18,8 @@
 #define CARTOGRAPHER_SENSOR_INTERNAL_VOXEL_FILTER_H_
 
 #include <bitset>
-#include <unordered_set>
 
+#include "absl/container/flat_hash_set.h"
 #include "cartographer/common/lua_parameter_dictionary.h"
 #include "cartographer/sensor/point_cloud.h"
 #include "cartographer/sensor/proto/adaptive_voxel_filter_options.pb.h"
@@ -58,7 +58,7 @@ class VoxelFilter {
   Eigen::Array3i GetCellIndex(const Eigen::Vector3f& point) const;
 
   float resolution_;
-  std::unordered_set<KeyType> voxel_set_;
+  absl::flat_hash_set<KeyType> voxel_set_;
 };
 
 proto::AdaptiveVoxelFilterOptions CreateAdaptiveVoxelFilterOptions(