diff --git a/.gitmodules b/.gitmodules
index c28bb72c3..55ea47e8d 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -23,3 +23,6 @@
 [submodule "third_party/wide-integer"]
 	path = third_party/wide-integer
 	url = ../../ckormanyos/wide-integer.git
+[submodule "third_party/parallel-hashmap"]
+	path = third_party/parallel-hashmap
+	url = ../../greg7mdp/parallel-hashmap.git
diff --git a/libgringo/CMakeLists.txt b/libgringo/CMakeLists.txt
index b9a60a717..c0c935d63 100644
--- a/libgringo/CMakeLists.txt
+++ b/libgringo/CMakeLists.txt
@@ -126,7 +126,7 @@ set(source
 # ]]]
 
 add_library(libgringo STATIC ${header} ${source})
-target_link_libraries(libgringo PUBLIC libpotassco libreify tsl::ordered_map tsl::hopscotch_map tsl::sparse_map tl::optional mpark::variant math::wide_integer)
+target_link_libraries(libgringo PUBLIC libpotassco libreify tsl::ordered_map tsl::hopscotch_map tsl::sparse_map tl::optional mpark::variant math::wide_integer grep::phmap)
 target_include_directories(libgringo
     PUBLIC
     "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>"
diff --git a/libgringo/src/symbol.cc b/libgringo/src/symbol.cc
index 0da667d31..c33bf7f01 100644
--- a/libgringo/src/symbol.cc
+++ b/libgringo/src/symbol.cc
@@ -24,10 +24,12 @@
 
 #include <algorithm>
 #include <cstring>
+#include <mutex>
+
+#include <parallel_hashmap/phmap.h>
+
 #include <gringo/hash_set.hh>
 #include <gringo/symbol.hh>
-#include <iterator>
-#include <mutex>
 
 #ifdef _MSC_VER
 #pragma warning(disable : 4200) // nonstandard extension used: zero-sized array in struct/union
@@ -79,28 +81,22 @@ String toString(uint64_t rep) { return String::fromRep(ptr(rep)); }
 
 template <class T> struct UniqueConstruct {
   public:
-    using Set = hash_set<T, typename T::Hash, typename T::EqualTo>;
+    using Set =
+        phmap::parallel_flat_hash_set<T, typename T::Hash, typename T::EqualTo, std::allocator<T>, 6, std::mutex>;
 
     template <class U> static T const &construct(U &&x) {
-        // TODO: in C++17 this can use a read/write lock to not block reading threads
-        size_t hash = typename T::Hash{}(x);
-        std::lock_guard<std::mutex> g(mutex_);
-        auto it = set_.find(x, hash);
-        if (it != set_.end()) {
-            return *it;
-        }
-        return *set_.insert(T{std::forward<U>(x), hash}).first;
+        size_t raw_hash = typename T::Hash{}(x);
+        size_t mixed_hash = phmap::phmap_mix<sizeof(size_t)>()(raw_hash);
+        return *set_.lazy_emplace_with_hash(x, mixed_hash,
+                                            [&x, raw_hash](auto const &ctor) { ctor(std::forward<U>(x), raw_hash); });
     }
 
   private:
-    static Set set_;          // NOLINT
-    static std::mutex mutex_; // NOLINT
+    static Set set_; // NOLINT
 };
 
 template <class T> typename UniqueConstruct<T>::Set UniqueConstruct<T>::set_; // NOLINT
 
-template <class T> typename std::mutex UniqueConstruct<T>::mutex_; // NOLINT
-
 template <class T, class U> T const &construct_unique(U &&x) {
     return UniqueConstruct<T>::construct(std::forward<U>(x));
 }
@@ -112,8 +108,9 @@ class MSig {
     using Cons = std::pair<String, uint32_t>;
 
     struct Hash {
+        using is_transparent = void;
         size_t operator()(MSig const &sig) const { return sig.hash_; }
-        size_t operator()(Cons const &sig) const { return hash_mix(get_value_hash(sig)); }
+        size_t operator()(Cons const &sig) const { return get_value_hash(sig); }
     };
     struct EqualTo {
         using is_transparent = void;
@@ -190,9 +187,10 @@ class MFun {
     using Cons = std::pair<Sig, SymSpan>;
 
     struct Hash {
+        using is_transparent = void;
         size_t operator()(MFun const &a) const { return a.fun_->hash(); }
         size_t operator()(Cons const &a) const {
-            return hash_mix(get_value_hash(a.first, hash_range(begin(a.second), end(a.second))));
+            return get_value_hash(a.first, hash_range(begin(a.second), end(a.second)));
         }
     };
     struct EqualTo {
@@ -285,9 +283,10 @@ class String::Impl {
 class String::Impl::MString {
   public:
     struct Hash {
+        using is_transparent = void;
         size_t operator()(MString const &str) const { return str.str_->hash(); }
-        size_t operator()(char const *str) const { return hash_mix(strhash(str)); }
-        size_t operator()(StringSpan const &str) const { return hash_mix(strhash(str)); }
+        size_t operator()(char const *str) const { return strhash(str); }
+        size_t operator()(StringSpan const &str) const { return strhash(str); }
     };
     struct EqualTo {
         using is_transparent = void;
diff --git a/third_party/CMakeLists.txt b/third_party/CMakeLists.txt
index 57bdd993a..ad7d86a4c 100644
--- a/third_party/CMakeLists.txt
+++ b/third_party/CMakeLists.txt
@@ -30,3 +30,7 @@ add_library(mpark::variant ALIAS variant)
 add_library(wide_integer INTERFACE)
 target_include_directories(wide_integer INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/wide-integer>)
 add_library(math::wide_integer ALIAS wide_integer)
+
+add_library(phmap INTERFACE)
+target_include_directories(phmap INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/parallel-hashmap>)
+add_library(grep::phmap ALIAS phmap)
diff --git a/third_party/parallel-hashmap b/third_party/parallel-hashmap
new file mode 160000
index 000000000..8442f1c82
--- /dev/null
+++ b/third_party/parallel-hashmap
@@ -0,0 +1 @@
+Subproject commit 8442f1c82cad04c026e3db4959c6b7a5396f982a