diff --git a/src/libexpr/json-to-value.cc b/src/libexpr/json-to-value.cc
index 33ab55ee9..21074bdd8 100644
--- a/src/libexpr/json-to-value.cc
+++ b/src/libexpr/json-to-value.cc
@@ -42,7 +42,7 @@ class JSONSax : nlohmann::json_sax<json> {
             auto attrs2 = state.buildBindings(attrs.size());
             for (auto & i : attrs)
                 attrs2.insert(i.first, i.second);
-            parent->value(state).mkAttrs(attrs2.alreadySorted());
+            parent->value(state).mkAttrs(attrs2);
             return std::move(parent);
         }
         void add() override { v = nullptr; }
diff --git a/src/libexpr/symbol-table.hh b/src/libexpr/symbol-table.hh
index dee7369e8..be12f6248 100644
--- a/src/libexpr/symbol-table.hh
+++ b/src/libexpr/symbol-table.hh
@@ -70,6 +70,8 @@ public:
 
     auto operator<=>(const Symbol other) const { return id <=> other.id; }
     bool operator==(const Symbol other) const { return id == other.id; }
+
+    friend class std::hash<Symbol>;
 };
 
 /**
@@ -133,3 +135,12 @@ public:
 };
 
 }
+
+template<>
+struct std::hash<nix::Symbol>
+{
+    std::size_t operator()(const nix::Symbol & s) const noexcept
+    {
+        return std::hash<decltype(s.id)>{}(s.id);
+    }
+};
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index fdc6c84c4..257da1d2d 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -494,11 +494,11 @@ void Value::mkBlackhole()
 
 #if HAVE_BOEHMGC
 typedef std::vector<Value *, traceable_allocator<Value *>> ValueVector;
-typedef std::map<Symbol, Value *, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, Value *>>> ValueMap;
+typedef std::unordered_map<Symbol, Value *, std::hash<Symbol>, std::equal_to<Symbol>, traceable_allocator<std::pair<const Symbol, Value *>>> ValueMap;
 typedef std::map<Symbol, ValueVector, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, ValueVector>>> ValueVectorMap;
 #else
 typedef std::vector<Value *> ValueVector;
-typedef std::map<Symbol, Value *> ValueMap;
+typedef std::unordered_map<Symbol, Value *> ValueMap;
 typedef std::map<Symbol, ValueVector> ValueVectorMap;
 #endif