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/gringo/shared_mutex.hh b/libgringo/gringo/shared_mutex.hh
new file mode 100644
index 000000000..0a48a7b5b
--- /dev/null
+++ b/libgringo/gringo/shared_mutex.hh
@@ -0,0 +1,154 @@
+#pragma once
+
+#if defined(_WIN32) || defined(_WIN64)
+#define GRINGO_PLATFORM_WINDOWS 1
+#else
+#define GRINGO_PLATFORM_WINDOWS 0
+#endif
+
+#if !GRINGO_PLATFORM_WINDOWS && (defined(__unix__) || defined(__APPLE__))
+#define GRINGO_PLATFORM_POSIX 1
+#else
+#define GRINGO_PLATFORM_POSIX 0
+#endif
+
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <shared_mutex>
+
+#if GRINGO_PLATFORM_WINDOWS
+#define WIN32_LEAN_AND_MEAN
+#define NOMINMAX
+#include <Windows.h>
+#elif GRINGO_PLATFORM_POSIX
+#include <pthread.h>
+#endif
+
+#ifndef PHMAP_DISABLE_MIX
+#define PHMAP_DISABLE_MIX 1
+#endif
+#include <parallel_hashmap/phmap_base.h>
+
+namespace Gringo {
+
+#if GRINGO_PLATFORM_POSIX
+namespace Detail {
+
+inline void check_pthread_call(char const *fun, int code) noexcept {
+    if (code != 0) {
+        std::fprintf(stderr, "fatal: %s failed: %s\n", fun, std::strerror(code));
+        std::abort();
+    }
+}
+
+} // namespace Detail
+#endif
+
+class SharedMutex {
+  public:
+    SharedMutex() noexcept {
+#if GRINGO_PLATFORM_WINDOWS
+        InitializeSRWLock(&srw_);
+#elif GRINGO_PLATFORM_POSIX
+        Detail::check_pthread_call("pthread_rwlock_init", pthread_rwlock_init(&rw_, nullptr));
+#endif
+    }
+
+    ~SharedMutex() noexcept {
+#if GRINGO_PLATFORM_POSIX
+        Detail::check_pthread_call("pthread_rwlock_destroy", pthread_rwlock_destroy(&rw_));
+#endif
+    }
+
+    SharedMutex(const SharedMutex &) = delete;
+    SharedMutex &operator=(const SharedMutex &) = delete;
+    SharedMutex(SharedMutex &&) = delete;
+    SharedMutex &operator=(SharedMutex &&) = delete;
+
+    void lock_shared() noexcept {
+#if GRINGO_PLATFORM_WINDOWS
+        AcquireSRWLockShared(&srw_);
+#elif GRINGO_PLATFORM_POSIX
+        Detail::check_pthread_call("pthread_rwlock_rdlock", pthread_rwlock_rdlock(&rw_));
+#else
+        mtx_.lock_shared();
+#endif
+    }
+
+    bool try_lock_shared() noexcept {
+#if GRINGO_PLATFORM_WINDOWS
+        return TryAcquireSRWLockShared(&srw_) != 0;
+#elif GRINGO_PLATFORM_POSIX
+        return pthread_rwlock_tryrdlock(&rw_) == 0;
+#else
+        return mtx_.try_lock_shared();
+#endif
+    }
+
+    void unlock_shared() noexcept {
+#if GRINGO_PLATFORM_WINDOWS
+        ReleaseSRWLockShared(&srw_);
+#elif GRINGO_PLATFORM_POSIX
+        Detail::check_pthread_call("pthread_rwlock_unlock", pthread_rwlock_unlock(&rw_));
+#else
+        mtx_.unlock_shared();
+#endif
+    }
+
+    void lock() noexcept {
+#if GRINGO_PLATFORM_WINDOWS
+        AcquireSRWLockExclusive(&srw_);
+#elif GRINGO_PLATFORM_POSIX
+        Detail::check_pthread_call("pthread_rwlock_wrlock", pthread_rwlock_wrlock(&rw_));
+#else
+        mtx_.lock();
+#endif
+    }
+
+    bool try_lock() noexcept {
+#if GRINGO_PLATFORM_WINDOWS
+        return TryAcquireSRWLockExclusive(&srw_) != 0;
+#elif GRINGO_PLATFORM_POSIX
+        return pthread_rwlock_trywrlock(&rw_) == 0;
+#else
+        return mtx_.try_lock();
+#endif
+    }
+
+    void unlock() noexcept {
+#if GRINGO_PLATFORM_WINDOWS
+        ReleaseSRWLockExclusive(&srw_);
+#elif GRINGO_PLATFORM_POSIX
+        Detail::check_pthread_call("pthread_rwlock_unlock", pthread_rwlock_unlock(&rw_));
+#else
+        mtx_.unlock();
+#endif
+    }
+
+  private:
+#if GRINGO_PLATFORM_WINDOWS
+    SRWLOCK srw_;
+#elif GRINGO_PLATFORM_POSIX
+    pthread_rwlock_t rw_;
+#else
+    std::shared_timed_mutex mtx_;
+#endif
+};
+
+} // namespace Gringo
+
+namespace phmap {
+
+template <> class LockableImpl<Gringo::SharedMutex> : public Gringo::SharedMutex {
+  public:
+    using mutex_type = Gringo::SharedMutex;
+    using Base = LockableBaseImpl<Gringo::SharedMutex>;
+    using SharedLock = std::shared_lock<mutex_type>;
+    using ReadWriteLock = typename Base::ReadWriteLock;
+    using UniqueLock = std::unique_lock<mutex_type>;
+    using SharedLocks = typename Base::ReadLocks;
+    using UniqueLocks = typename Base::WriteLocks;
+};
+
+} // namespace phmap
diff --git a/libgringo/src/symbol.cc b/libgringo/src/symbol.cc
index 0da667d31..30ea57b0d 100644
--- a/libgringo/src/symbol.cc
+++ b/libgringo/src/symbol.cc
@@ -24,10 +24,15 @@
 
 #include <algorithm>
 #include <cstring>
+
+#ifndef PHMAP_DISABLE_MIX
+#define PHMAP_DISABLE_MIX 1
+#endif
+#include <parallel_hashmap/phmap.h>
+
 #include <gringo/hash_set.hh>
+#include <gringo/shared_mutex.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 +84,24 @@ 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>, 4,
+                                              Gringo::SharedMutex>;
 
     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 hash = set_.hash(x);
         auto it = set_.find(x, hash);
-        if (it != set_.end()) {
+        if (it != end(set_)) {
             return *it;
         }
-        return *set_.insert(T{std::forward<U>(x), hash}).first;
+        return *set_.emplace_with_hash(hash, std::forward<U>(x), hash).first;
     }
 
   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));
 }
@@ -112,6 +113,7 @@ 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)); }
     };
@@ -190,6 +192,7 @@ 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))));
@@ -285,6 +288,7 @@ 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)); }
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