diff --git a/Triangulation_2/include/CGAL/Triangulation_2/internal/Polyline_constraint_hierarchy_2.h b/Triangulation_2/include/CGAL/Triangulation_2/internal/Polyline_constraint_hierarchy_2.h
index 7c26bc7cb795..b7afe8a7fc4c 100644
--- a/Triangulation_2/include/CGAL/Triangulation_2/internal/Polyline_constraint_hierarchy_2.h
+++ b/Triangulation_2/include/CGAL/Triangulation_2/internal/Polyline_constraint_hierarchy_2.h
@@ -21,6 +21,15 @@
 #include <map>
 #include <set>
 #include <list>
+
+#include <boost/version.hpp>
+#if BOOST_VERSION >= 108100
+#  include <boost/unordered/unordered_flat_map.hpp>
+#  define CGAL_USE_BOOST_UNORDERED 1
+#else // BOOST before 1.81.0
+#  include <unordered_map>
+#endif
+
 #include <CGAL/Skiplist.h>
 #include <CGAL/assertions.h>
 
@@ -134,24 +143,6 @@ class Polyline_constraint_hierarchy_2
     }
   };
 
-  class Pair_compare {
-    Compare comp;
-
-  public:
-    Pair_compare(const Compare& comp) : comp(comp) {}
-
-    bool operator()(const Edge& e1, const Edge& e2) const {
-      if(comp(e1.first, e2.first)) {
-        return true;
-      } else if((! comp(e2.first, e1.first)) && //  !less(e1,e2) && !less(e2,e1) == equal
-                comp(e1.second, e2.second)) {
-        return true;
-      } else {
-        return false;
-      }
-    }
-  };
-
   class Context {
     friend class Polyline_constraint_hierarchy_2<T,Compare,Point>;
   private:
@@ -171,8 +162,11 @@ class Polyline_constraint_hierarchy_2
   typedef typename Context_list::iterator Context_iterator;
 
   typedef std::set<Constraint_id>           Constraint_set;
-  typedef std::map<Edge, Context_list*,
-                   Pair_compare>            Sc_to_c_map;
+#if CGAL_USE_BOOST_UNORDERED
+  typedef boost::unordered_flat_map<Edge, Context_list*, boost::hash<Edge>> Sc_to_c_map;
+#else
+  typedef std::unordered_map<Edge, Context_list*, boost::hash<Edge>> Sc_to_c_map;
+#endif
   typedef typename Constraint_set::iterator C_iterator;
   typedef typename Sc_to_c_map::const_iterator    Sc_iterator;
   typedef Sc_iterator Subconstraint_iterator;
@@ -186,7 +180,7 @@ class Polyline_constraint_hierarchy_2
 public:
   Polyline_constraint_hierarchy_2(const Compare& comp)
     : comp(comp)
-    , sc_to_c_map(Pair_compare(comp))
+    , sc_to_c_map()
   { }
   Polyline_constraint_hierarchy_2(const Polyline_constraint_hierarchy_2& ch);
   Polyline_constraint_hierarchy_2(Polyline_constraint_hierarchy_2&&) = default;
@@ -290,7 +284,7 @@ template <class T, class Compare, class Point>
 Polyline_constraint_hierarchy_2<T,Compare,Point>::
 Polyline_constraint_hierarchy_2(const Polyline_constraint_hierarchy_2& ch)
   : comp(ch.comp)
-  , sc_to_c_map(Pair_compare(comp))
+  , sc_to_c_map()
 {
   copy(ch);
 }