diff --git a/src/rules/simulation/PhysicalDomain.cpp b/src/rules/simulation/PhysicalDomain.cpp
index 32d3aa228..8096caf1f 100644
--- a/src/rules/simulation/PhysicalDomain.cpp
+++ b/src/rules/simulation/PhysicalDomain.cpp
@@ -640,7 +640,7 @@ std::list<LocatedEntity*> PhysicalDomain::getObservingEntitiesFor(const LocatedE
 class PhysicalDomain::VisibilityCallback : public btCollisionWorld::ContactResultCallback
 {
     public:
-        std::set<BulletEntry*> m_entries;
+        std::unordered_set<BulletEntry*> m_entries;
 
         btScalar addSingleResult(btManifoldPoint& cp, const btCollisionObjectWrapper* colObj0Wrap, int partId0, int index0, const btCollisionObjectWrapper* colObj1Wrap,
                                  int partId1, int index1) override
diff --git a/src/rules/simulation/PhysicalDomain.h b/src/rules/simulation/PhysicalDomain.h
index 09750e07b..1e8b898fe 100644
--- a/src/rules/simulation/PhysicalDomain.h
+++ b/src/rules/simulation/PhysicalDomain.h
@@ -31,6 +31,7 @@
 #include <tuple>
 #include <array>
 #include <set>
+#include <unordered_set>
 #include <BulletCollision/CollisionDispatch/btGhostObject.h>
 
 namespace Mercator {
@@ -130,11 +131,11 @@ class PhysicalDomain : public Domain
             /**
              * Set of entries which are observing by this.
              */
-            std::set<BulletEntry*> observedByThis;
+            std::unordered_set<BulletEntry*> observedByThis;
             /**
              * Set of entries which are observing this.
              */
-            std::set<BulletEntry*> observingThis;
+            std::unordered_set<BulletEntry*> observingThis;
 
             btVector3 centerOfMassOffset;