diff --git a/src/search/landmarks/landmark.h b/src/search/landmarks/landmark.h
index bca8024547..7473f61672 100644
--- a/src/search/landmarks/landmark.h
+++ b/src/search/landmarks/landmark.h
@@ -3,7 +3,7 @@
 
 #include "../task_proxy.h"
 
-#include <set>
+#include <unordered_set>
 
 namespace landmarks {
 class Landmark {
@@ -31,8 +31,8 @@ class Landmark {
     bool is_true_in_goal;
     bool is_derived;
 
-    std::set<int> first_achievers;
-    std::set<int> possible_achievers;
+    std::unordered_set<int> first_achievers;
+    std::unordered_set<int> possible_achievers;
 
     bool is_true_in_state(const State &state) const;
 };
diff --git a/src/search/landmarks/landmark_cost_partitioning_algorithms.cc b/src/search/landmarks/landmark_cost_partitioning_algorithms.cc
index 807346007c..b669b7e785 100644
--- a/src/search/landmarks/landmark_cost_partitioning_algorithms.cc
+++ b/src/search/landmarks/landmark_cost_partitioning_algorithms.cc
@@ -22,7 +22,7 @@ CostPartitioningAlgorithm::CostPartitioningAlgorithm(
     : lm_graph(graph), operator_costs(operator_costs) {
 }
 
-const set<int> &CostPartitioningAlgorithm::get_achievers(
+const unordered_set<int> &CostPartitioningAlgorithm::get_achievers(
     const Landmark &landmark, bool past) const {
     // Return relevant achievers of the landmark according to its status.
     if (past) {
@@ -60,7 +60,7 @@ double UniformCostPartitioningAlgorithm::get_cost_partitioned_heuristic_value(
     for (auto &node : nodes) {
         int id = node->get_id();
         if (future.test(id)) {
-            const set<int> &achievers =
+            const unordered_set<int> &achievers =
                 get_achievers(node->get_landmark(), past.test(id));
             if (achievers.empty())
                 return numeric_limits<double>::max();
@@ -93,7 +93,7 @@ double UniformCostPartitioningAlgorithm::get_cost_partitioned_heuristic_value(
     for (auto &node : nodes) {
         int id = node->get_id();
         if (future.test(id)) {
-            const set<int> &achievers =
+            const unordered_set<int> &achievers =
                 get_achievers(node->get_landmark(), past.test(id));
             bool covered_by_action_lm = false;
             for (int op_id : achievers) {
@@ -120,7 +120,7 @@ double UniformCostPartitioningAlgorithm::get_cost_partitioned_heuristic_value(
         // TODO: Iterate over Landmarks instead of LandmarkNodes
         int id = node->get_id();
         assert(future.test(id));
-        const set<int> &achievers =
+        const unordered_set<int> &achievers =
             get_achievers(node->get_landmark(), past.test(id));
         double min_cost = numeric_limits<double>::max();
         for (int op_id : achievers) {
@@ -216,7 +216,7 @@ double OptimalCostPartitioningAlgorithm::get_cost_partitioned_heuristic_value(
     for (int lm_id = 0; lm_id < num_cols; ++lm_id) {
         const Landmark &landmark = lm_graph.get_node(lm_id)->get_landmark();
         if (future.test(lm_id)) {
-            const set<int> &achievers =
+            const unordered_set<int> &achievers =
                 get_achievers(landmark, past.test(lm_id));
             if (achievers.empty())
                 return numeric_limits<double>::max();
diff --git a/src/search/landmarks/landmark_cost_partitioning_algorithms.h b/src/search/landmarks/landmark_cost_partitioning_algorithms.h
index 833ead053f..a4a6cdf80d 100644
--- a/src/search/landmarks/landmark_cost_partitioning_algorithms.h
+++ b/src/search/landmarks/landmark_cost_partitioning_algorithms.h
@@ -5,7 +5,7 @@
 
 #include "../lp/lp_solver.h"
 
-#include <set>
+#include <unordered_set>
 #include <vector>
 
 class OperatorsProxy;
@@ -21,7 +21,7 @@ class CostPartitioningAlgorithm {
     const LandmarkGraph &lm_graph;
     const std::vector<int> operator_costs;
 
-    const std::set<int> &get_achievers(
+    const std::unordered_set<int> &get_achievers(
         const Landmark &landmark, bool past) const;
 public:
     CostPartitioningAlgorithm(const std::vector<int> &operator_costs,
diff --git a/src/search/landmarks/landmark_sum_heuristic.cc b/src/search/landmarks/landmark_sum_heuristic.cc
index 07cfc0068e..ac687a56b2 100644
--- a/src/search/landmarks/landmark_sum_heuristic.cc
+++ b/src/search/landmarks/landmark_sum_heuristic.cc
@@ -44,7 +44,7 @@ LandmarkSumHeuristic::LandmarkSumHeuristic(const plugins::Options &opts)
 }
 
 int LandmarkSumHeuristic::get_min_cost_of_achievers(
-    const set<int> &achievers) const {
+    const unordered_set<int> &achievers) const {
     int min_cost = numeric_limits<int>::max();
     for (int id : achievers) {
         OperatorProxy op = get_operator_or_axiom(task_proxy, id);
diff --git a/src/search/landmarks/landmark_sum_heuristic.h b/src/search/landmarks/landmark_sum_heuristic.h
index 6a9a21ab08..c47af5b090 100644
--- a/src/search/landmarks/landmark_sum_heuristic.h
+++ b/src/search/landmarks/landmark_sum_heuristic.h
@@ -10,7 +10,8 @@ class LandmarkSumHeuristic : public LandmarkHeuristic {
     std::vector<int> min_first_achiever_costs;
     std::vector<int> min_possible_achiever_costs;
 
-    int get_min_cost_of_achievers(const std::set<int> &achievers) const;
+    int get_min_cost_of_achievers(
+        const std::unordered_set<int> &achievers) const;
     void compute_landmark_costs();
 
     int get_heuristic_value(const State &ancestor_state) override;