diff --git a/cth/CMakeLists.txt b/cth/CMakeLists.txt
index adb41cc..3c69052 100644
--- a/cth/CMakeLists.txt
+++ b/cth/CMakeLists.txt
@@ -15,6 +15,11 @@ target_compile_options(cth INTERFACE $<$<CXX_COMPILER_ID:MSVC>:/Zc:preprocessor>
 cth_pkg_target_find_package(cth boost_pfr CONFIG REQUIRED)
 target_link_libraries(cth INTERFACE Boost::pfr)
 
+# cth/data/hash_map.hpp includes gtl/phmap.hpp, gtl installs no cmake package
+cth_pkg_add_find_modules(cmake/Findgtl.cmake)
+cth_pkg_target_find_package(cth gtl MODULE REQUIRED)
+target_link_libraries(cth INTERFACE gtl::gtl)
+
 target_link_libraries(cth INTERFACE $<$<AND:$<CXX_COMPILER_ID:GNU,Clang>,$<NOT:$<PLATFORM_ID:Windows>>>:stdc++exp>)
 
 # tests
diff --git a/cth/cmake/Findgtl.cmake b/cth/cmake/Findgtl.cmake
new file mode 100644
index 0000000..f1b5900
--- /dev/null
+++ b/cth/cmake/Findgtl.cmake
@@ -0,0 +1,12 @@
+# gtl installs only its headers and no cmake package, this module creates the target gtl::gtl
+include(FindPackageHandleStandardArgs)
+
+find_path(gtl_INCLUDE_DIR gtl/phmap.hpp)
+find_package_handle_standard_args(gtl REQUIRED_VARS gtl_INCLUDE_DIR)
+
+if(gtl_FOUND AND NOT TARGET gtl::gtl)
+    add_library(gtl::gtl INTERFACE IMPORTED)
+    target_include_directories(gtl::gtl SYSTEM INTERFACE "${gtl_INCLUDE_DIR}")
+endif()
+
+mark_as_advanced(gtl_INCLUDE_DIR)
diff --git a/cth/incl/cth/algorithm/combine.hpp b/cth/incl/cth/algorithm/combine.hpp
index fbc33a6..0a168d0 100644
--- a/cth/incl/cth/algorithm/combine.hpp
+++ b/cth/incl/cth/algorithm/combine.hpp
@@ -1,4 +1,5 @@
 #pragma once
+#include "cth/data/hash_map.hpp"
 #include "cth/io/log.hpp"
 #include "cth/meta/ranges.hpp"
 
@@ -9,7 +10,6 @@
 #include <queue>
 #include <ranges>
 #include <span>
-#include <unordered_map>
 #include <unordered_set>
 #include <vector>
 
@@ -26,7 +26,7 @@ template<mta::range2d_over_cpt<CPT(std::equality_comparable)> Rng>
 [[nodiscard]] auto unique_combine(Rng const& selections) {
     using T = std::remove_cvref_t<mta::range2d_value_t<Rng>>;
     using node_t = T const*;
-    using node_selection_map_t = std::unordered_map<node_t, size_t>;
+    using node_selection_map_t = dt::flat_hash_map<node_t, size_t>;
     static constexpr node_t EMPTY_NODE = nullptr;
 
     auto const selectionCount = std::ranges::size(selections);
diff --git a/cth/incl/cth/data/hash_map.hpp b/cth/incl/cth/data/hash_map.hpp
new file mode 100644
index 0000000..632cf6c
--- /dev/null
+++ b/cth/incl/cth/data/hash_map.hpp
@@ -0,0 +1,16 @@
+#pragma once
+#include <gtl/phmap.hpp>
+
+namespace cth::dt {
+/**
+ * @brief hash map without pointer stability, a rehash moves the elements
+ */
+template<class Key, class T, class... Args>
+using flat_hash_map = gtl::flat_hash_map<Key, T, Args...>;
+
+/**
+ * @brief hash set without pointer stability, a rehash moves the elements
+ */
+template<class Key, class... Args>
+using flat_hash_set = gtl::flat_hash_set<Key, Args...>;
+}
diff --git a/cth/incl/cth/data/pool.hpp b/cth/incl/cth/data/pool.hpp
index 637a077..bdfb460 100644
--- a/cth/incl/cth/data/pool.hpp
+++ b/cth/incl/cth/data/pool.hpp
@@ -1,4 +1,5 @@
 #pragma once
+#include "cth/data/hash_map.hpp"
 #include "cth/io/log.hpp"
 #include "cth/meta/concepts.hpp"
 
@@ -7,7 +8,6 @@
 #include <functional>
 #include <memory>
 #include <ranges>
-#include <unordered_set>
 #include <vector>
 
 namespace cth::dt {
@@ -113,7 +113,7 @@ class pool {
 private:
     void resetActive() {
         if constexpr(HAS_RESET) {
-            std::unordered_set inactiveSet{std::from_range, _inactive};
+            flat_hash_set<T*> const inactiveSet{_inactive.begin(), _inactive.end()};
 
             for(auto& stored : _storage)
                 if(!inactiveSet.contains(&stored))
diff --git a/cth/incl/cth/data/thread_pool.hpp b/cth/incl/cth/data/thread_pool.hpp
index 1442b1a..24a9af2 100644
--- a/cth/incl/cth/data/thread_pool.hpp
+++ b/cth/incl/cth/data/thread_pool.hpp
@@ -1,10 +1,10 @@
 #pragma once
+#include "cth/data/hash_map.hpp"
 #include "cth/data/pool.hpp"
 
 #include <cstddef>
 #include <mutex>
 #include <thread>
-#include <unordered_map>
 
 namespace cth::dt {
 
@@ -91,7 +91,7 @@ class thread_pool {
 private:
     mutable std::mutex _mtx;
     pool_t _pool;
-    std::unordered_map<std::thread::id, T*> _bound{};
+    flat_hash_map<std::thread::id, T*> _bound{};
 
 public:
     /** total number of instances in the pool */
diff --git a/cth/incl/cth/data/union_find.hpp b/cth/incl/cth/data/union_find.hpp
index 91ff5c4..2bf96eb 100644
--- a/cth/incl/cth/data/union_find.hpp
+++ b/cth/incl/cth/data/union_find.hpp
@@ -2,8 +2,8 @@
 #include "cth/data/poly_vector.hpp"
 
 #include <cstddef>
-#include <unordered_map>
-#include <unordered_set>
+#include <limits>
+#include <vector>
 
 namespace cth::dt {
 /**
@@ -162,14 +162,16 @@ class union_find {
     [[nodiscard]] constexpr std::vector<group_t> all() {
         std::vector<group_t> result{};
 
-        std::unordered_map<index_type, size_t> groups{};
+        // group of each root, the roots are indices
+        constexpr size_t NO_GROUP = std::numeric_limits<size_t>::max();
+        std::vector<size_t> groups(size(), NO_GROUP);
 
         for(index_type idx = 0; idx < size(); idx++) {
-            auto const parent = find(idx);
-            if(auto groupIt = groups.find(parent); groupIt != groups.end())
-                result[groupIt->second].emplace_back(idx);
+            auto& group = groups[find(idx)];
+            if(group != NO_GROUP)
+                result[group].emplace_back(idx);
             else {
-                groups[parent] = result.size();
+                group = result.size();
                 result.emplace_back(1)[0] = idx;
             }
         }
diff --git a/cth/tests/src/data/thread_pool_test.cpp b/cth/tests/src/data/thread_pool_test.cpp
index 421938e..6a80648 100644
--- a/cth/tests/src/data/thread_pool_test.cpp
+++ b/cth/tests/src/data/thread_pool_test.cpp
@@ -1,5 +1,6 @@
 #include "cth/data/thread_pool.hpp"
 #include "cth/constants.hpp"
+#include "cth/data/hash_map.hpp"
 #include "test.hpp"
 
 #include <atomic>
@@ -7,7 +8,6 @@
 #include <latch>
 #include <string>
 #include <thread>
-#include <unordered_set>
 #include <vector>
 
 
@@ -157,7 +157,7 @@ DATA_TEST(thread_pool, distinct_ids_get_distinct_resources) {
     EXPECT_TRUE(tp.exhausted());
 
     // three owners hold three distinct resources
-    std::unordered_set<int*> refs{&a, &b, &c};
+    cth::dt::flat_hash_set<int*> refs{&a, &b, &c};
     EXPECT_EQ(refs.size(), 3);
 }
 
@@ -292,7 +292,7 @@ DATA_TEST(thread_pool, concurrent_acquire_gives_each_thread_a_distinct_resource)
     EXPECT_TRUE(tp.exhausted());
 
     // every thread received a distinct resource, none null
-    std::unordered_set<int*> unique{acquired.begin(), acquired.end()};
+    cth::dt::flat_hash_set<int*> unique{acquired.begin(), acquired.end()};
     EXPECT_EQ(unique.size(), THREADS);
     EXPECT_FALSE(unique.contains(nullptr));
 }
diff --git a/cth_net/src/websocket_server.cpp b/cth_net/src/websocket_server.cpp
index 0b68e50..4181504 100644
--- a/cth_net/src/websocket_server.cpp
+++ b/cth_net/src/websocket_server.cpp
@@ -3,13 +3,14 @@
 #include "cth/net/exception.hpp"
 #include "websocket_connection_state.hpp"
 
+#include "cth/data/hash_map.hpp"
+
 #include <hv/WebSocketServer.h>
 #include <hv/hsocket.h>
 
 #include <atomic>
 #include <deque>
 #include <mutex>
-#include <unordered_map>
 #include <utility>
 #include <vector>
 
@@ -22,7 +23,7 @@ struct websocket_server::impl {
     std::uint16_t boundPort{};
 
     mutable std::mutex mutex;
-    std::unordered_map<hv::WebSocketChannel*, std::shared_ptr<dev::websocket_connection_state>>
+    dt::flat_hash_map<hv::WebSocketChannel*, std::shared_ptr<dev::websocket_connection_state>>
     connections;
     std::deque<std::shared_ptr<dev::websocket_connection_state>> accepted;
     os::fence readyFence;
diff --git a/lib/cth_cmake b/lib/cth_cmake
index 09a6c5d..79a6191 160000
--- a/lib/cth_cmake
+++ b/lib/cth_cmake
@@ -1 +1 @@
-Subproject commit 09a6c5d02421cba58fcf33bc3e4fd3d4fb646002
+Subproject commit 79a619176696cebe86d5d56059681f369a8b50e6
diff --git a/vcpkg.json b/vcpkg.json
index cb8f175..c7df85b 100644
--- a/vcpkg.json
+++ b/vcpkg.json
@@ -6,6 +6,7 @@
     "boost-pfr",
     "glm",
     "gtest",
+    "gtl",
     "stb",
     "asio",
     "libhv"